I would think that is because unlike Juggernauts/Guardians who really have to spec into a proper DPS form, Powertechs/Vanguards get both BH/Trooper DPS forms out of the gate. I guess they had to put some mechanic in there to stop them from being overpowered (especially hybrid builds).
On the subject of tanks, what is the deal with Assassin/Shadow tanks? Do they have some sort of equivalent to forms? Or do they just spec into passive tanking talents in their appropriate tree?