I drive in B 90% of the time. I'm sure the computer is programed to protect the motor and the batteries. For example, when I leave my house I drive down a hill, loosing 500 vertical feet. If the batteries are not fully charged, the car will regen heavily all the way down. But, if the batteries are fully charged, I get very little regen and have to burn up the real brakes instead. The computer is protecting the batteries from being over charged.
Further, have you seen the number of warnings and cautions in the owners' manual? Mitsubishi didn't miss an opportunity to tell you what not to do.
If they had any qualms about us driving in B, they would have put a caution or warning in the owners' manual.