Okay, Bak says:
*The equipment-types are difficult to be mastered because of their powerful strength, so they must be altered into weapons. The new shape restrains the power.
*Parasitic types synchronize with the "raw, unaltered form of Innocence" which means that they have to restrain that power themselves - so they are
And then he went on to tell Allen that he's still a human. Riiiiight...
Still doesn't explain what the hell happened to Rinari and her Innocence, and why she didn't die from overusing it when it's supposedly too much power for her to handle (oh, and, um, she should have drowned).