Yes, some of the time the theist is just acting disingenuously in order to get a reaction, but some really are stupid in thinking that atheism is a belief.
I've even come across arguments in the past where I've been told I'm not an atheist because I don't fit the classical, Greek definition. Well, this is the 21st century and language and meanings have moved on quite a bit since those times. If you applied this same reasoning to all words (you know, so you're behaving consistently and not just picking out one word to suit the point you're trying to make), then you'd be talking complete gibberish now.
Anyway, it's the same dumb "logic" I see time and time again - "You don't believe this therefore you must believe the opposite". No. Just NO! They see God as everything and since we don't believe that, then we must believe the opposite and believe in nothing. It's bonkers. All of these people are completely at home with the concept of not believing things, because they themselves don't believe in a myriad of things, yet somehow, when it comes to theism/atheism, some part of their brain switches off where they become blinded and oblivious to the idea that someone doesn't actually believe in the existence of gods. It's a great example of compartmentalisation.