Hello Chananya,
Here are some answers that may help.
JetBlue: With regards to the "ATC" option on JetBlue flights, there are some restrictions. Basically, the ATC option allows passengers to listen to the ATC conversations that the flightdeck is having. This would allow, for example, passengers to know when they are doing a maneuver such as entering a final descent or a base turn. This option, however, only works when the pilot has decided to turn it on. For example, on a regular flight, the ATC option should work for the whole time, but on a flight in which there is some sort of emergency or VIP encounter, the pilot may decide to turn off the ATC option as to not frighten or in any way worry the passengers aboard.
United Airlines: With regards to this change in inflight entertainment aboard United, I have less insight. I do propose the idea, though, that United Airlines may have changed their policy, for whatever reason, to make ATC unavailable to passengers. It could be a security thing, or for some other reason. I'm not sure. Otherwise, I surgest that you give United a call on their customer service number [1 (800) 864-8331], where I'm sure someone would be able to explain the change to you.
Hope this helps,
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