They effectively do dissipate the extra energy already - as heat, from the physical brakes. If they can't charge the battery faster than .25g of breaking would provide, there's no reason to put increased stress on the engine (it's much more expensive than a physical braking system).

Some electric trains actually do this, they dump the energy as heat in a bank of resistors. The motor on a tesla is on the rear axel, but most braking power is dissipated from the front axel due to the weight transfer. The question is if you also had a front axel motor (like the Model X), how much regen could you do. I remember reading somewhere that induction motors don't regen that well at low RPMs, hence you need to supplement with friction brakes. Anyone with a better understanding care to comment; would it be possible to build a car with no friction brakes, just 100% regen? I suppose you could always apply a counter torque to the motor.

Keep in mind the regenerative braking is only on the rear wheels, and it is the front wheels that usually do most of the braking.