Well, arresting someone who just "won" an important election (why this election was important i have no fucking idea) would probably be a bad look. Bringing her in for questioning accomplishes the same task and he could have arrested her while being questioned anyways so whatever.My only problems with them are two-fold:
1. Robyn wasn't on the arrest warrant from the get go.
2. The Ace-Ops girls trying to kill team RWBY when their orders were explicitly to arrest them.
On the other hand, ace ops and team whatever both basically have super powers and would probably have to get pretty damn close to killing them in order to bring them in. Of course they had a person who could have easily incapacitated them without harming them too much, but that character had to be stupid and not use their OP ability in the perfect situation in order for the plot to happen.