It being plastic means it will deform and slip off before you can apply any massively damaging torque, even if it is in a metal BB spanner (try taking off a particularly seize BB with one of those plastic adaptors). Not that it will allow to set a specific torque. Same as when the preload crank cap tool is made of plastic too.
I think it's plastic for cost/manufacture reasons. Shimano BBs can be tightened to up to 50 Nm. Which is a lot. Crank cap only up to 1.5 Nm, which is why the tool only has finger grips and is made out of much softer plastic
It being plastic means it will deform and slip off before you can apply any massively damaging torque, even if it is in a metal BB spanner (try taking off a particularly seize BB with one of those plastic adaptors). Not that it will allow to set a specific torque. Same as when the preload crank cap tool is made of plastic too.