Because people are not here on this forum necessarily to revert - calling them a non-believer is an insult to someone who believes in something, regardless of whether our point of view. They are here with their own beliefs. Good for them. If they are inspired by something they see here to take a step towards Islam, alhamdulilah. If they don't but simply come away with one less stereotype about Moslems, alhamdulilah.
Non-Moslem is more than adequate as a description is it not? Nothing gets lost in the meaning, does it?! And not getting their back up in the first place is adab on our part, right?
Think back to your own days of kuffr. Was the true path revealed to you by someone getting your back up? Or was it witnessing the beauty of the true religion being practised with manners and patience?