I also have no idea why they would forbid that, provided of course the kayaks are properly secured. The added weight is relatively small, as well--maybe a hundred pounds or so. It would be very easy to add more weight by putting stuff in the cargo space of the vehicle.
If they extend past the front of the towed vehicle, one would be well advised to verify adequate clearance in tight turns. Likewise, if they extend beyond the back of the towed vehicle, one needs to be aware of that as well (and of whatever applicable restrictions there may be on overall vehicle combination length).