Why can't I reassign primary and secondary buttons

[anselmk-l1ve]

Hi there,
due to a special condition with my hands (and years of being used to use regular mice) I have the buttons setup in a very special way on my 5-button-left-handed-vertical-mouse (that doesn't have special driver software for mac unfortunately). When I want to use that mouse on the mac I got for work, the mapping is not configurable in the way I need it (or in any proper way).

When I found this mac-mouse-fix I was hoping it could fix this problem, but apparently I am not allowed to reassign primary and secondary buttons. Is there a good and comprehensible reason for that or is it just another apple-knows-best-what-you-need-thing?

Thanks,
Anselm