BTW, since by definition EVERYTHING in UO belongs NOT to the players, but to EA Mythic, technically wagers of in-game gold between players aren't changing the current or final ownership of the wagered currency (be it an in-game lotto for housing spots or in-game wagers in a PvP arena). After all, EA does not sell the gold to the players the way online poker sites sell additional chips to players, and the wagers appear to be gold only. The only money/gold transactions are done by parties not expressly authorized to do so, and in fact are discouraged.
Anyone trying to invoke gambling law as a deterrant to an in-game gold wagering system, therefore, would face an uphill battle in most nations. Comparisons to online poker sites are invalid on multiple points as noted above.
Furthermore, much as poker is considered a "game of Skill" not a game of chance, in some jurisdictions, PvP would be considered even more so a game of skill, and of course does NOT have any kind of amateur or professional association (and could hardly be considered a sport, as defined in sports gambling laws). Otherwise, picking up weapons and ammo in FPS multiplayer games would be considered "gambling".