I was under the impression that you were going to eventually be getting her something that would let her jump right back to her post whenever there's danger, at which point if she made the request and had done her duty well she would be allowed to have that much freedom. But until she has some way of doing that, it kind of defeats the purpose of having a stationed guard if said guard isn't there to actually handle threats. And as there have been actual threats against Redgate made by Tarz (which Korra will ICly be made aware of at the end of the trial), this really isn't a time when she'd be given leave to just run off and do whatever she wants.

This is meant to be a punishment, not a job, so it isn't necessarily going to work well with what Korra wants to do. As time passes and she earns some trust back it can certainly be arranged for her to have more freedom, but this whole arrangement has only just been set up and there is reason right now why that guard post is necessary. If Korra takes off to join in a fight and especially without even asking for permission, Sara would have to see that as her abandoning her post when she was most needed and he'd probably have to reinstate her bounty.