In the situation that law enforcement seizes a phone (as evidence or inventory) and inserts it into a faraday sleeve, they sure dont want someone remotely accessing, signaling, texting, pinging, polling, hacking, or wiping the phone, so yes, a smartphone might be ON and actively searching for tower signals and wifi signals while it's in a their 'chain of custody' faraday sleeve.
But for most people, if you insert your smartphone into a faraday sleeve while it is in normal mode, (meaning it is actively looking or pinging for a tower signal, and/or a wifi or bluetooth signal) why would you leave it in that mode, knowing the battery will drain quicker, AND you might drive right past a tower that the phone can very briefly 'see' and it will try to respond by transmitting a signal back to the tower...and it might even be 'seen' by the tower.
In other words, how does one verify the faraday sleeve is actually doing it's job UNLESS you test it by driving up fairly close to a tower? Then if you do get a text, for example, then you know at that point you had better turn it OFF while in the sleeve if you want it unreachable and invisible to the phone network.
And then again, if you plan to turn it off or use airplane mode while in the sleeve, then you really dont need the sleeve to begin with. Unless you are putting a spare or burner smartphone in the sleeve, turned OFF, as a spare for that 1 in 10 million chance that an N-EMP happens within 500 miles of you and maybe you might need a spare phone for taking pictures or maybe GPS use when you discover the world has gone SHTF.
Now YES...you can buy or build EMP bags and cases for OTHER electronic devices, such as generators, 2 way radios, cameras, c-paps, portable am/fm radios, etc etc, but NONE of these items are constantly trying to ping the local cell tower, so cutting off ALL exterior RF makes more sense.