UL Panelboard Marking Guide '07
page 5
Voltage Rating
6. The basic voltage markings must be visible without disturbing wiring and other interior parts. . A panelboard designed and intended for use only on a supply circuit involving two different potentials (for example, 120/240 volts, three-wire; or 208Y/120 volts, three-phase, four-wire) is so marked.
It would be my guess that the reason for the restriction and the use of the word "only" is because of concern with the MWBC situation. . There are electricians everywhere that would land the hots on side by side breakers and not give it another thought. . Doubled neutral currents is a huge concern.
You'll have to check the panel label to know for sure but I hope UL has handled this one properly and is preventing legal installations of a single hot to multiple bus bars. . I'm sure it will still continue to be done even if not legal. . But I hope their panel marking is keeping it out of legal installations.
David