Synthesized Radios
Yes you can choose the channel you use, but having done that, it's exactly the same as if you only had one channel. If you choose, say, channel 52, you use that channel in the same way as if you had no choice and Ch 52 was your only channel.
You still have to put your channel pin up, and others cannot then use it.
All the synthesiser does is allow you to use a channel you can specify.
Synthesisers are legal in the USA. What you can't do is change the crystal in your transmitter yourself, without then sending it off to the manufacturer who will fine tune it to that chosen channel.
Synthesisers do the fine tuning job for you.
-David C.