I've heard two plausible explanations:
1. Tire ballast + backhoe weight creates the potential for a more severe roll-over that could not be stopped by the ROPS. A liability issue for the manufacturer. This came from the dealer that sold me my tractor and backhoe. Concept makes sense to me, as does the likelihood overcautious corporate attorneys had something to do with it.
2. The possibility of damaging the backhoe if it were used to lift the back end of a tire-ballasted tractor for repositioning. That's something commonly done with full-sized TLB's, I don't do it with my setup, even without ballast in the tires. You can say the same things about this concept: There's a certain amount of sense to it, and it probably came from some lawyer.
Are either of these officially from the manufacturer? Who knows!