I recently got a 3 months of HBO because Comcast screwed up the bill and was shocked to see that the SD versions of the films are all FS. I was pretty shocked since I would think by now that the standard would be WS. Trying to watch something in FS (even free) is almost intolerable.

It is nice to get HBO on-demand though and catch up on True Blood and Rome for free!

Do other cable/satellite providers do FS on HBO, Showtime, Cinemax, etc.?

I never noticed since I refuse to watch the non-HD premium channels. Judging by what I what I see on DTV now (at least on the flagship channels):

HBO - FS
SHO - letterboxed
STARZ - FS

Why would you think HBO would letterbox their SD channel when they show pretty much everything on the HD channel in 1.78? HBO is a fill the screen kind of network that doesn't really care about OAR.

The providers have absolutely nothing to do with this. They just transmit what HBO sends them. And on SD channels, it's still 4x3 as it's always been. Those are mainly intended for people still watching 4x3 SD TV's, so FS makes sense (or at least as much as it ever has).

If you have an HDTV and want widescreen, just watch the HD version of the channel.