Quite simply - some mothers just don't feel the need, want or desire to be away from their little ones as they enjoy being with them and don't want a break.
A first outfit is like a my first Christmas outfit. This doesn't bother me but I know some DIL's I've spoken to, this has been a little bit of an issue as it tends to be a "first" - a new mother gets to buy her little one their first Christmas outfit. Its meant to be an exciting experience and some DILs want this for themselves, naturally. I don't personally see it as a big issue but I do know some that do and are bothered by it.
I think that girls tend to be more lenient of their mothers and probably tolerate more than they would from their MIL which is unfair. Both should be treated fairly in terms of their relationships. A relationship between a mother and a MIL is not the same, however. A woman is probably more likely to trust and feel secure with their own mother watching their LO. Its because they have been raised by their mother and know the parenting style. Also they might find it easier to tell them how to look after LO.
I think in terms of a power struggle sometimes it is. A LOT of DILs think MILs try and continue being the "authority" figure if you like, despite DIL being a peer and an adult. The PIL believe they get to ovverule DIL and make decisions, or feel entitled to make choices about what baby eats, sleeps, naps, drinks, where baby goes, parental choices. They probably don't mean to but often come across controlling and overbearing, while the MIL isn't understanding what they've done to offend DIL. Some DILs are insecure as are some MILS. They're both just people.
Yes i'm going to be a MIL one day and hope I can be a good MIL (as I bet some of you are!)
Unfortunately some people are just not so nice, doesn't matter if they're girls, boys, adults, old, sick, smart, professional. People are just people with a standard amount of variation.