Yeah, you are right. It isn't quite that simple. The thing is, that 7800 GPU (listed for Arma 2) came out eight years ago. The IGPs (integrated graphics processors) that ship with current Intel Core CPUs may perform rather poorly by todays standards, but they certainly can keep up with the graphics cards from so long ago. Expectedly, todays IGP's will run Arma 2 just fine.
Contrast that to BF3, which is based on one of the most demanding graphics engines ever built. BF3 was designed explicitly to push the envelope in terms of graphics quality. It was deliberately built to redefine high end PC graphics. Expecting that to run on an Intel IGP, the lowest-end graphics hardware currently available, is a little ridiculous. It's like buying a Ferrari and strapping that to two horses, instead of ever turning on the engine. It just doesn't make much sense. Anyone buying that kind of game really should get the graphics hardware to do it justice.