Continuity mistake: When Obi-Wan lines up his commandeered cannon for a second shot and Waxer and Boil load in another charge, they grab the ammunition from the same exact position at the top of a stack that they took the first ammunition charge, used for Obi-Wan's first shot with the commandeered gun. (00:21:09)
Continuity mistake: Obi-Wan drops his lightsaber during the run-in with the gutkurrs, and Cody is explicitly shown returning it to him after the creatures are contained. However, when Obi-Wan initially releases his control on the creatures, the weapon is visible on his belt. (00:17:24)
Continuity mistake: When Obi-Wan, Waxer and Boil take cover behind some rocks on the way in to Nabat, in a close-up of Obi-Wan, the armour markings of the trooper next to him are missing. (00:04:37)
Answer: I guess that 1) the King assumed that Ventress' droids were meant to capture the Jedi, not kill him outright. When she yells at OOM-224 to shoot Yoda, she evidently failed that expectation. 2) Sending out a bunch of Droidekas after it is plain that Master Yoda has clearly won is not a sign of good sportsmanship, either.