Regen is factored into their calculations of battery life. While no one but Tesla knows the actual details of their recharging strategy, I suspect that the regen also selectively charges certain cells/strings of cells to reduce the overall amount of charge cycles. Hypothetically, maybe the car drains one string down to a low level and then each time the regen kicks in, it goes to that discharged string until it is full then that string will be put back into use and another discharged string will be the recipient of regen.