Continuity mistake: After Grievous has received R3's message, he assures the dismembered R2 that he will not come to harm. We see R2's head lying on the dissection table with his electronic eye dangling from a cable. When Grievous stands up again, just before his MagnaGuards enter, R2 has his eye back in place. It's a brief shot, so you'll have to be quick to catch that glitch.
Continuity mistake: Ahsoka escapes from General Grievous by cutting his right hand off. When Grievous jumps into his starfighter, he suddenly has his hand back. (00:18:11)
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.