A couple reasons.
One good reason is that airlines are extraordinarily paranoid about the internet. If they know "John Franklin, pilot for Junkt Airways" is talking about safety infractions or crazy things they've done in the cockpit, they're afraid that it'll get scooped by the media. A little anonymity goes a long way on the internet in terms of being able to express your views and opinions.
Secondly, everyone and his uncle knows who I work for if they've followed the website for any amount of time. Over the past few months, I've been bullied and threatened by a subsidiary of my employer because I don't trumpet support of their program and they wanted to quell any and all dissent. See the 'update' post at the top of General Topics for more information.