As far as I can ascertain, Triumph is less than overjoyed about car tires or trailer hitches.
If you have a car tire on the bike or a trailer hitch and you take the bike in for service of any kind and a RO is made out, in the comments section, the technician will note the prescence of a car tire or trailer hitch on the bike.
I would presume that the notation has to do with any future warranty claims much like the addition of a sidecar voids the warranty.
In as much as a fair number of Rockets seem to suffer from mechanical problems (mine included) I consider it advantageous not to incite Triumph to deny a warranty claim for any reason, whether it's a car tire, trailer hitch or some other add-on that might, in their eyes, nullify the warranty.