It's a question of side-effects. Whatever number is "full time" will result in gaming of the law by cutting hours to just under that number. Obamacare sought to sidestep that by going down to 30, but it wasn't low enough. On the other hand, if they dropped it to say... 10... that would be crazy, too. No one would ever get a true part-time job.
Thank you both for your insights.