As a few people have said, lots of these places have insurance that doesn't cover members of the public. And the fact that there was a Morrisons 5 minutes walk away, and it seemed they made a lot of effort to help her makes this all the more ludicrous.
If I saw that review (and their response,) no way would I not go to this cafe.
@isabellerossignol
Although I do think there should be some sort of system whereby councils give rates reductions or something like that to business who will allow the public to use their toilets. There just aren't very many public toilets available and it's very limiting for people who have bladder or bowel problems.
Yep this. Great idea. 
I live near a little town - it's 10 minutes drive away from me, and that town and the other 2 that are close-ish (15-17 minutes drive each,) all have very little toilet facilities.
My little town has a small Tesco that has toilet facilities, and a little toilet block that is 2 minutes walk from the big Tesco, but in the main high street (15 minutes walk from the Tesco,) there is nothing. 50 or 60 shops in the high street,, and no public loos for 15 minutes walk. Except in Costa and 3 cafes there. But you have to be a customer/buy a drink or something to eat to be able to use it.
Costa has a panel on the door where you have to enter a 6 digit code that you get when you buy something, and it changes daily. The 3 cafes have their loos right at the back past the counter, so you can't really sneak past. There are half a dozen pubs but again, they don't like or want randoms using the loos if they're not a customer.
The other 2 towns I was referring to are similar. Only one set of public loos, and you can often be 15 to 20 minutes walk from them, and the cafes and pubs etc, don't usually like people using their loos and sometimes will refuse. So businesses and companies being given incentives to allow people to use their loo(s.) Or provide an extra one for the public.
Costa by the way, used to allow people in, but kids/teens kept going in there pissing about and causing a mess, so they locked them off. Can't say I blame them really. This is this the reason why most places refuse people, because of vandalism and theft. Not sure what to do about it really. The few really do spoil it for the many. 
I do agree with the people saying there should be more toilet facilities, because some people do have conditions that mean they need the loo 5 minutes ago!!!