I tested this by assisting another player, vs setting to various pettank options. Works fine when assisting another player!
Sub CheckRoles controls whether mounts are allowed for hunterpettank, pettank, and pullerpettank. The lines that involved where /varset MountOn 0 are lines 7224, 7240, 7258.
If it is by design to not allow a mount, I don't understand it.
Note. I "fixed" the problem by commenting out line 7240. Note this only affects pettank, the role I use.