Because some mower companies have been sued by idiots who injured others or themselves by reversing over people, todlers, hoses, electrical wires , etc etc .
All modern US mowers have reverse cut out and most ZTRs now days cut out when both lap bars are in reverse .
The courts seem to think you can legislate out stupidity .
Mowers made specifically for anywhere else in the world do not have the MIR cut outs
The Australian export models & European export models we get here do not have MIR , the US home models do .