I agree that it is the 'having' which is the problem. Always wanting to see children without their mum seems fairly common, but makes the mum feel like an incubator, and reinforces the idea that she is 'other', particularly when it is done in the early early weeks and months.
For example, comments like 'I can't wait until mummy wises up and gives you a big bottle, so you can come away to my house, with me' tend to stick in the head for the longest time. projecting massively, sorry
Been there and no, it doesn't make me inclined to involve the people who do it. I have to make a real and conscious effort to consider them over the people who are happy to see me, and spend time with me, as well as my children. 
I do think it is a in law thing, because the person who has actually had the baby, actually given birth to it, is not their genetic family. There is a bit of a disconnect inherent in the relationship even before there are children. The baby becomes a really big deal because it is their relation, and in the excitement the mum becomes an afterthought, and the seeds of resentment are sown. Being charitable about it I would say in most cases it isn't even a conscious thing, but the effects of it can last for ever.