Hey, OP, no surprise or 'wondering' about it at all to me.
Men / Sons and their Mothers....
It is very, very, very, common.....
However, the other poster is right.
It IS his family. For better or worse.
Unless it is a toxic situation (not just - these people are horrid and annoying) you really need to respect the fact that he does have family and that he will want to see them.
As long as there are boundaries, such as 'no more weekly obligated barbeques'... then you should really kind of suck-it-up. Of course, you have a right to your feelings! And, you don't have to really like your inlaws... But, just sayin'.
PS: Been there done that with my inlaws... This post is from somebody who knows how you feel. I did actually have to outright refuse to see my inlaws when FIL was living, because his behavior towards me was nasty and unacceptable and I refused to subject our young son to that....
Guess what - we are still somewhat 'obligated' to drive an hour to see DH's mother, and listen to self-centered comments, and eat her nasty cooking, every single Sunday. I SOOOOO look forward to when we can actually make our excuses and have an occasional weekend without seeing her.