That's precisely the problem with lube on the spindle though. The lube makes the crank slide farther onto the spindle at a given torque. So, holding torque equal, the crank will slide farther down a lubed spindle than an unlubed one, possibly leading to deformation of the contact surfaces.

Think about the different mechanics involved in bolting on a crank versus all the other bolts you tighten on your bike. Typically, you're just squeezing the broad surface of one part directly against the broad surface of another. (Think, stem and handlebars, or seat post and seat tube.) But, with the crank, you're sliding it farther and farther onto the taper of the spindle. Totally different mechanics involved.