Note that this is about a temporary injunction directing Starbucks to rehire these workers until the courts come to a decision about whether or not the firing was legal. It is not a decision about whether or not the firing was legal.
With that said,
they allowed a television news crew into the Starbucks cafe after hours to talk about the union campaign
I find it entirely plausible that Starbucks would fire anyone who lets a news crew into the cafe after hours, even if it had nothing to do with unionizing.
It’s important to note it was a meeting to discuss a union and they brought in a tv crew to record it. Which is fairly normal. Many union meetings have outside staff coming in; lawyers or union reps come to mind.