Well, I guess I'll be the odd one here. I do think it's strange. I don't know how they've treated him in the past, so that would be part of the answer.
My mom has done the same thing for years. She usually does something smallish-range for my birthday, but I usually get a check. Dh is lucky if he gets a card.
For Christmas, she gets the kids a few things, gets my dh and sister's dh (our dh's are married, by the way) one thing. But, she LOADS us down with gifts. Granted, they're not expensive, but she walks in with arm loads of presents. Even the kids go
So she basically treats my sister and me like we're still 12.
Now, on the other hand, dh's parents have ALWAYS spent the same amount of $$ on everyone. Once you marry in, you're included. They either decrease the total amount they spend or increase it to accommodate the new person. His grandparents do the same thing too.
I will treat everyone equally when my kids grow up. I do not want to make my children's spouses feel uncomfortable. If I can't afford to do much, I might consider sending them a joint gift, but I will NOT leave anyone out. That only leads to hurt feelings.
I don't think your parents did this intentionally...it was, more than likely, an oversight.