It's an alien land. The laws of physics are different there than in Randland. It would have made an incredibly obnoxious read if RJ had listed how every rule in that land worked and why channeling wouldn't have freed them. It gets annoying that people can't just take this at face value. I've seen this brought up at least 100 times now. They could stop Lanfear and Moiraine because they could because they could because they could. Why do we have to have an explanation for every last thing?

I beg to differ that it wouldn't stop Lanfear from leaving. The 'Finn hid easily from Thom, Mat, and Noal when they wanted to. They could have refused to ever show their faces and she would have starved that way.