I had a very similar experience lately at my work place. I posted a question about my company here and they found it and where concerned. Like yours it was not slandering the company in anyway shape or from. We to have a union and they said the samething your union did. I am not sure where you live but I got legal advise and it was money well spent to learn your rights as a worker. I did the same thing you did by posting on my own time using my own computer in this public forum which the company has no controll over. I opted to remove my post as I just dont want conflict with work but legally I did not have to. My new moto is

"Do your work as thoroughly as you can to the best of your ability and only do what your told to do"

If you are concerned for your job I would recommend get legal advise for an attorney that specialises in workers rights.