I get your frustration OP. I sense we may not know the whole story here.
Yes parents can buy their adult children gifts, but there seems to be a sense in which they are still regarding him as young, free and single. They rarely see their grandchildren and choose not to bring them gifts, but buy one for your dh. If they acknowledged you all as a family and bought him gifts too, that would be different, but you seem to feel ignored/overlooked, yes?
They suggest that he is working hard and needs a break, but do not acknowledge that you are doing so too, so again they seem to be overlooking you.
And they buy him a gift which will encourage him to play alone - it's not a wii for family fun, but a PS3, which he may well be playing alone on - again, not a problem for him to have a hobby and relax alone, but with two tiny children and a wife, spending time playing Fifa or CoD or whatever, may not be your idea of his priority at the moment.
If Im right then the key question is can you discuss with your dh how you perceive his parents - does he feel as if they are seeing him as a little boy or as a man with a family of his own now - and how does he feel about the way that they feel about you? If you are hurt, looking for a stronger relationship with them, or looking for his support then you need to talk to him about it (If he's not busy playing CoD now of course!)
Best of luck