It only caused no end of trouble for those whose roots were in the command-line interface and who were accustomed to this_kind_of_naming_convention.  I suppose it's still widely used on the PC side to maintain compatibility with those who use Linux, as somebody said above, or who still have an old 286 PC I suppose. But mostly because it's decades-old habit. I know a lot of people who still put two spaces after every period and 3 spaces at the start of each paragraph, dating back to primitive  typewriters.

As far as I can remember, Macs never had the "no spaces" and "8 characters" limits that the DOS and WIndows programmers originally placed on themselves so long ago. If Poser was an original Mac product, perhaps it only adopted the old-style PC naming structure when it was released for the PC. It takes me back to the 1980s whenever Poser requires adherence to this structure (it drops the 2nd word if you name a material with 2 words in the Group Editor, for example).