i would say that while it's rude, try not to be mad.
sounds like you know the groom, and in most cases, it's probably the bride who would do anything about thank you notes. i know from my wedding, if i had not done everything on my own, no thank you notes would have gone out. terrible. but he just does not see the value in the energy, time, cost of doing thank you notes.
and then there's my sister, the most unorganized person, who amazingly WROTE out all the thank you cards, put names on the envelopes, never addressed them, and never mailed them. 5 years ago. it was just more than she could handle. my mom got lots of phone calls ("just checking to see if she actually got my gift...") - very embarrassing for her.
so, if they are anything like my family - try not to be too harsh on them!