Because they don't justification to take someone down. If the Party could take down high ranking Party members like Bo Xilai or massively popular celebs like Fan Bingbing or foreign citizens like that Swedish publisher or a group of people like the million Uighurs, they hardly need an elaborate scheme like this to take someone into custody.
I guess it depends on the image they want to project and the message they want to send. Sometimes there is a political end to a Government's posturing. Sometimes it's more than the end they're looking for, sometimes they need to set the scene first to control the narrative in a way that sells the story they want for their own political gain.
It's the game of wits from Princess Bride. They might want to take someone out, so you can't choose the wine in front of you. But, they must know you know that, so you can't choose the wine in front of them. But they may be trying to protect their image, and if so, you can't choose the wine in front of them either.
You're overthinking it.