Why? Because of the people who might buy the OS. And because that group may not be as "few" as you suspect, and because that group probably includes a decent number of people who are using the "pro" machines and don't want to piss them off. To take your philosophy to an extreme, why support any machines beyond the ones currently for sale? After all, the other ones are all "dead", right?
It makes sense only if people actually upgrade their machines.
It doesn't make sense if it annoys a big enough portion of the current user base and they go with another option instead of upgrading.
Without knowing more details it's hard to say which would be greater. There's something to be said for keeping customers happy, but I guess that's more long term thinking instead of going for the quickest buck.
To be honest, I have been looking at buying a new mac fairly soon, but if they're dropping support for machines this quickly, it makes me more inclined to put it off as long as I can, to see what ends up happening with their support.