If it merely slowed down, it would still be driving illegally. Everywhere I've driven in the USA, if there is a stopped school bus in front of you, or on the other side of the road, all traffic on the road must stop and remain stopped until the school bus retracts the sign and resumes driving.
I mean, it can be programmed in any way, so why not program it to follow both rules: If the sign is extended, then stop. Else if the sign is not extended, slow down.
Worst case, you needlessly slow down around a roadside parked schoolbus.
Best case, you save a kid's life.