Hiya Woody! Sorry for the delay in responding.
I'm not so sure that it's quite as care-less a mistake as, say, "it's for its", but then it would help if the last example that I can recall of "-ble for -bly" hadn't appeared alongside (and thus compounded!) "it's for its" on a webpage advertising ELT cert courses (INTESOL's, to be precise!).
Anyway, my theory is that people are reading the -e of -ble as acute (a la French)!
