Taxi cameras, why?

Ok, so the government is making it compulsory for taxi drivers to put cameras in there cars right?  They are doing this because some taxi drivers have been tragically injured – so its a safety issue.

But if its in the driver’s interest to have the camera, and they are the only ones getting a benefit from it, then surely they would only do it if the benefit of putting in the camera exceeded the cost.  In which case, regulating for them to put cameras in is either pointless, or forces taxi drivers to do something where the cost exceeds the benefit – and so is suboptimal.

What am I missing?  I must be missing something here, so just point it out to me and I’ll be happy :D

Eric Crampton discusses here.