You know, the reason Standard cars aren't able to change the rims/wheels, apart from the obvious reason that they didn't really spend much time on Standard cars at all, might have to do with 2 things.
One, if they had the same amount of options as they did in GT4, the for whatever reason extremely limited, Premium-selection would appear even more limited compared to it perhaps.
The other reason might have to do with pit-stops somehow, Standard cars are shown in chase cam with the wheels they're fitted with lying ready to be changed in the pit-lane whilst pit-stops in Premium cars can only be seen from within the interior, no wheels are seen.
Maybe they couldn't implement all modelled changeable wheels as it would be too much data maybe, or whatever, so each Standard car has a fixed set of wheels accurately displayed (and therefore programmed) in pit-stops and custom-rims on Premium cars conveniently aren't shown in pit-stops.
Why the Premium wheel-selection is so limited I don't know, however since these wheels are actually modelled now, instead of previous GT's graphical textures, makes it rather obvious that the selection would be much smaller than the one in GT4.
Basically the whole Premium car issue over again, more detail taking more time and thus less quantity but much higher quality although I doubt they also took months each to create....
The fact the whole selection isn't even available on all Premium cars baffles me to be honest so yes more wheels please.