I believe it was because Snow had ordered them not to, as he believed he was defusing the situation by sending her to another Hunger Games (and eliminating her in it). Suppressing the rebellions elsewhere wasn't working - it was making things worse.
At this stage in the films (and novels), the rebellions are becoming more pronounced and widespread, and Snow and the peacekeepers have thus far responded with brutality - but it hasn't quelled them in any way. In fact, they've gotten worse.
Katniss realises as much in the Catching Fire novel in Chapter 6, when she notices the riots in District 8 on the mayor's television:
I'm leaving the room when a beeping noise catches my attention. I turn
back to see the screen of the television go black. Then the words
“UPDATE ON DISTRICT 8” start flashing. Instinctively I know this is
not for my eyes but something intended only for the mayor. I should
go. Quickly. Instead I find myself stepping closer to the television.
An announcer I've never seen before appears. It's a woman with graying
hair and a hoarse, authoritative voice. She warns that conditions are
worsening and a Level 3 alert has been called. Additional forces are
being sent into District 8, and all textile production has ceased.
They cut away from the woman to the main square in District 8. I
recognize it because I was there only last week. There are still
banners with my face waving from the rooftops. Below them, there's a
mob scene. The square's packed with screaming people, their faces
hidden with rags and homemade masks, throwing bricks. Buildings burn.
Peacekeepers shoot into the crowd, killing at random.
I've never seen
anything like it, but I can only be witnessing one thing. This is what
President Snow calls an uprising.
So we know that the brutal suppression of these small acts of defiance have not been successful. The people, spurred on by Katniss' defiance, are rapidly fighting back.
However, with the 75th Repeating, Snow is able to send Katniss and Peeta to another Hunger Games. District 12 shows defiance as you mention - but why suppress this? The suppression elsewhere has failed, and Katniss and Peeta are going to be killed in the Games. So why risk inflaming the crowd and sparking even more rebellions, when they could be killed off?
Doing this would result in the loss of the instigators of this defiance, hopefully crush the spirits of those that are rebelling elsewhere and remind the every person in every district that anyone standing up to the Capitol will be crushed.
Ultimately, I feel this all comes back to how wary Snow is of Katniss and how quickly he realises the symbol of defiance she represents.