Yeah, I'd be sending regrets on that one.
I mean, yeah, this would turn me off too, but we have to assume the parents-to-be have no idea that this is the way their parents decided to invite people. The parents to be didn't send the evite, the grandparents did, so I don't know if I would be so quick to send regrets.
I honestly believe that 9 times out of 10, etiquette breeches like this happen simply because the person doesn't know any better-- not because the "offender" has some dastardly plan.
If this was a close friend of yours, you would honestly not attend because the invitation is "tacky"? (and trust me-- I am not disagreeing here-- I don't generally like that word- I prefer off-color)
Edited to add that after re- reading the OP-- I am not sure who sent the evite, but it appears that someone other than the parents to be did. Same commentary stands, though.