Damn it... I forgot to address that issue. I have mixed tire treads on my Deuce and I have heard it from HD and a custom shop that it's not "really" recommended. Matter of fact the rear tire is a radial and the front is bias. Which is considered another no-no.
Dunlop says that's a no no on their website which could be construed as a trade standard. They say nothing about mixing treads and neither does anybody else (that I can find). This tells me that there is no trade standard, study, accident investigation or any other expert opinion stating that mixing treads is the least bit unsafe.
That said I still defend his right to have the policy, much as I would defend his right to beat his own head against the wall he he chooses to do so.