Some of it's for the same reason that un-punk folks still buy vinyl: bigger pictures/nicer artwork and a warmer, more intimate sound. MDC sounds like they're in my living room when played on my Rykocut through Sound Dynamic RTS-3s.
There's also an effort to support/promote vinyl by many of the established punk labels. At the record store I shop at in Baltimore, domestic new releases usually cost $8.99 on vinyl or $11.99 on CD. Euro imports are usually $13.99 on vinyl and $15.99 on CD. (And vinyl definitely is NOT cheaper to manufacture.)
Until very recently, you had to own a record player if you wanted to listen to earlier punk music. More and more older stuff is re-released on CD (and vinyl) now. But five years ago, if you wanted to hear 70s and 80s punk groups (outside of more famous bands like the Sex Pistols, Dead Kennedys, Black Flag, etc...)you had to buy it used, on vinyl.
I'm sure some of it's tradition too. Did you know that a lot of punk groups still release singles/45s/7"s?
There's also an unlikely punk-audiophile trend emerging. I'm guessing someone must be using their marketing noodle by trying to appeal to punks, collectors and vinyl-philes at the same time. The Simply Vinyl re-issues of the Clash catalog are a good expample. A month ago, I bought an 180 gram reissue of the Dead Boy's "Young, Loud and Snotty". It's stikes me a weird pairing, but I'm happy as hell to reap the benefits!
