with my son, the first couple times (he was around 18 months or so), when he started getting agressive, we gave lots of warnings and corrections (when it was daddy or me, or the cats). when i was sure he KNEW that it was wrong and did it anyway, that's when we started time outs. i rarely have to give them to him now (he is 2 1/2). maybe...once a week? my sitter did a lot of loving redirection and correction, and she actually did not start timeouts until after he had had a couple at home. (those would be the two minute kind with lots of explanations beforehand, and hugs and apologies afterwards) like i said, we rarely have to worry about him lashing out in anger now. his timeouts now are mostly for not listening to mommy, since he has so many better things to do now!
for the record i would be highly upset with a sitter that treated my son like that. you didn't mention if he was happily playing in the pack and play or crying the entire time (maybe the sitter didn't go into detail?) but at the very least i would let her know that's unacceptable, and keep an eye on it.