Because there's more to it than just that quote, such as:

“We certainly talked to the mayor about this. It was not safe for us to make any sort of arrests on those individuals in that large crowd,” Peterson continued. “The best thing, from my perspective, was what did occur and direct those vehicles to leave the area, which essentially de-escalated things.”

It could (very likely) be complete bullshit, or maybe this decision did actually come from the mayor's office for genuine de-escalation purposes. At any rate, providing the complete context behind the police's decision (as described in this article anyway) would have required copy-pasting several paragraphs, and I was trying to keep my reply as succinct as I could for visibility reasons.

Moreover, the question of "why did they do that" is a detour from the specific point I was trying to make, which was that the commenter I replied to made it sound like Lang had not been arrested at all, a misconception I wanted to dispel (because in this era of rampant misinformation, the truth needs to be illuminated wherever possible, and as everyone knows, no one actually reads the article).