I don't know for sure why Alltrax did it this way. I could say that it might be because that prevents battery damage from trying to regen into a fully-charged battery, but that would be pure speculation.
Hm. Odd reason if so. The old Elec-traks just regen away, no big deal there.Also this is probably not too needed because we use T105's. If you overcharge a T105 for a minute it will gas. No big deal, add water. If you overcharge an AGM it will gas. Big deal; you can't water. Plus as the batter voltage climbs the regen will be less and less effective anyway.
Also with a shunt motor you can control the regen by simply dropping the field voltage. Less voltage=less regen. Simple cutout to build.
Maybe the problem is the FETs can't take reverse voltage for significant periods of time?
Chris