How could they tell what is active? I mean really tell? It's difficult.
Besides, the more accounts they can report as active, the more "reach" they can sell to advertisers, and the higher the prices advertisers have to pay for that space.
Active users are probably the key to identifying fake and inactive users. There are likely to be patterns of use on Facebook from active users which apply themselves irrespective of whether a given user is a power user, casual user, etc. Adding friends, sending messages, checking other people's walls, having other people check their wall.
Anyone who doesn't conform to this pattern to within about 10% either side (not enough use / too much use) is probably a fake or spam account.
Okay, agreed that it can be done with some margin of error. I think 5% is fair.
But if 1 out of 20 "fake" accounts that are active are closed down by mistake, that's millions of pissed off customers and customer support emails. In other words, a massive headache.
Besides, the more accounts they can report as active, the more "reach" they can sell to advertisers, and the higher the prices advertisers have to pay for that space.