I think it's possibly because those same children were half raised by THEIR grandparents so assume the same will follow.
It works both ways though, my mil has provided some childcare for us since retiring, we accepted reluctantly because nursery couldn't offer us the days we needed when I returned to work.
We ensure it's minimal, that she only has 1 of the 2 children on the day and everything is here for her to make life as easy as possible. I have just dropped a day and eldest starts school in August so thankfully we can stop. She'll hopefully still help out in the odd emergency if free. However my motivation is more that she overfeeds them crap so incredibly my eldest has gained significant weight since she started having him a day a week.
I don't mind a treat but we're talking 2 chocolate bars, a bag of crisps, a bag of mini cheddars, lunch at McDonald's, 3 fruit shoots over a working day level of excess. Normal day with us he has maybe a bag of crisps or mini cheddars and a cup of squash, sweets and chocolate are huge treats and he doesn't look for them, even on the days she has him she's shoving them at him not him asking. We've spoken and spoke. To her but she doesn't see the excess, she's obese herself.
If I could have him in paid childcare the entire time I work I would and if I could temporarily drop another day u til he starts school I would but neitherseens to be possible so I grit my teeth and say thank you.
Anyway, yes, it's mad to me that people are so entitled but honestly I don't know a single parent grandparent relationship where I see that entitlement. All my peers have parents desparaoto help out where they have to limit the time for reasons like ours or mostly the grandparents just pop in for a visit once every couple of weeks to be made tea and waited on while they take photos with the kids and head off for another fortnight