I know that schools are under pressure from OFSTED, and many of them are tootling along on a 'good' grade, not great, not excellent.
So it doesn't take much to drop them to a 'requires improvement' grade.
I am actually in agreement with the OP. OFSTED rules state that the school year is 180 days, and your child MUST attend for 170 of them, or they are falling into the realms of 'low attendance.'
Stupid, stupid, stupid.
What if the child is ill for a week, and then has to have several 2 or 3 day periods off with a sore throat, bad cold, tummy bug, and OMG they have had MORE than the allocated 10 days off in a WHOLE SCHOOL YEAR?! 
My daughter is grown now (mid 20's,) but when she was between 5 and 12, I used to take her for a trip to the beach in school time a couple of times between April and June, and me and her and DH (her dad,) used to have a week off in late September/early October AND April every year to go on our family holiday.
She was told at school (at 9 y.o.) that in English, she has reading and writing age of 14, she has the maths skills level of a 13 y.o. and she is most definitely going to university.' (This was only just into the 2000's BTW...)
Her attendance was always good (around 90%) even with the days off. The school never complained, and didn't seem to make such a fuss then. They actually allowed 10 days off with no question. Upshot is, her having some 13-15 days off in the school year did not affect her education one single jot.
Fast forward to when she was at secondary school. My daughter had been doing very well for the first 2-3 years at school, and then part way though Year 9, (so when she was 13-14,) she and a friend of hers fell out.
The 'friend' got together with 5 or 6 other girls and my daughter was badly bullied at school for the second half of year 9, and the first half of year 10. Cyber bullying, stalking her, walking past the house and staring in, the mothers getting involved and joining in, and snubbing me, or giving me filthy looks. It was a nightmare and we had to get the police involved because of the harassment and the cyber bullying. My daughter was constantly low and depressed.
We contacted the school, and went to see them, and told them what was happening, and all they did was offer 'mediation' with the bullies, tell me that my daughter needs to learn to handle conflict better, and offer her counselling 
The bullies (and their mothers) just laughed, said we needed to 'grow up' and 'get a grip,' and we were mocked on social media, by the bullies AND their mothers.
It got to the point where my daughter wouldn't go into school, and I thought I was going to have to home school her. Her attendance dropped to 77%, which considering her life was being made a living hell, wasn't too bad actually!
The school then had the audacity to send a 'report' to the Education Authority about my daughter's 'poor attendance,' and I got a strongly worded letter from the Education authority, about my daughter's attendance, and how the school were concerned at how this 'low attendance' was going to affect her education.
Funny though, they didn't seem to give a flying fuck how the bullying was affecting her education (and her!) The Education Authority said that said if her attendance did not improve during the next half term, further action will be taken, and worst case scenario, me and her father could go to prison .... 
They said we need to contact the school to sort this matter, and try to work with the school to bring our daughter's attendance levels up again!!! 
I was incandescent with rage. I did not contact the school - I went to the Education Authority MYSELF, and gave them a nice little rundown of the reasons why my daughter's attendance had dropped, and the bullying she had had to endure for an entire school year.
They were furious, and said they were not aware of what had been happening, and contacted the school demanding to know what had been happening, and why this hadn't been mentioned in the report about my daughter's drop in attendance.
Cue shit hitting fan. The Education Authority went in all guns blazing to the school, and took our side. The school suddenly started showing a great deal of compassion for our daughter, the bullies were threatened with expulsion, (and their parents were chastised,) and the school brought in a new anti-bullying policy into the school, and brought in new cyber bullying rules and said anyone flouting them will be expelled.
The bullying stopped almost immediately (just a few snarky looks,) and my daughter is now a post grad who went to one of the top 10 universities in the UK, and is earning more than £50K in her mid 20's, she has her own home with her boyfriend, (who is also a high earner,) and has travelled to more than two dozen countries,. The bullies, unsurprisingly, have not done quite so well, and have achieved precisely nothing.
But yeah, going back to the OP. It's not the odd few days off and dropping to below 90% attendance that makes a child fail ... If they are failing, it will be for other reasons, like poor teaching, bullying, illness (that they cannot help,) or just that they are not academic. (And in a few cases, it's poor parenting...)
As a few posters have said, it's just the school worrying about their own OFSTED grades. Also, as a few posters have mentioned, it's a bit rich that the school will have inset days whenever it suits, and snow days, and send them home if it's too hot, (or too cold and the boiler has broken down!) Yet if you keep them off for a day to take them to a museum or to the beach or for a day in a big historic city, you get chastised for it.
One rule for them, another rule for parents. 