If the game releases on PS2, it's probably not meant to be a PS2-seller

You have to think about that guys: Even though the PSP might be as cheap as a new PS2 now, most people don't buy a PSP. Because they have a PS2 at home already. It's not like most people have to decide between PSP and PS2; they don't want to buy a new console only for WOPulse.
Most likely the port is meant as an addition to the PS2 owner's games collection.
And it fits there pretty well, as threre a plenty of people out there that still think the PS3 is too expensive (hell yeah it is) and have no need for a portable handheld.
If Sony can make a PS2 port of Pulse more or less easily (I don't know how much the coding differs) why shouldn't they release it? The PS2 isn't fully dead yet, remember that (I know a lot people who bought a new PS2 just in the last 1-2 years because of games like singstar).

The PS2 still lives in a lot of households, why not release games for it (if it's so easy)?