Continuity mistake: Besides Obi-Wan, Even Piell and Commander Cody, the exact number of people in Obi-Wan's group keeps changing.
Continuity mistake: After Obi-Wan's group has abseiled down the wall of the prison, two of the clone troopers in armour are carrying Separatist blasters instead of the Republic-issue blasters they had before and after. (00:09:38)
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.