I would imagine that the teachers in question teach some kind of vendor product. And they're competing directly with CompTIA certs. Everyone has bias.
You can get into a good job without any opensource if you find a company that doesn't use any. They do exist, though they are getting more rare as the big vendors port opensource tech instead of developing their own.
If you have strong ideological convictions regarding opensource, then all's good. But if not, then I suggest not limiting yourself to purely opensource techs. If you're looking for a job, there are rarely purely opensource jobs. Being able to say "I'm proficient at openoffice, but have worked a bit with ms office as well" is much better than saying "I've only ever used openoffice, sorry".