Having a little bit of interest about this issue my opinion is that people who tape shows via these methods engage in the same 'unethical(ness)' per se as unwarranted audio/video recording. Pretty simple. Unauthorized taping is unauthorized taping; probably 99% of all shows I attend/record do not allow audio recording.
The comparison someone made with saying how ALD/IEMs are why bands do not allow recording is not an argument; same thing bands say about audio/video recording and how it promotes selling of bootlegs which, really, does not happen all that much. Overwhelming of tapers would never sell their recordings. And, really, for the most part, anyone who torrents an ALD or IEM just makes it worse for everyone else.
Now would be a good time to make a note; ALD's tend to work within the very low MHZ range, for example a NADY 800 unit (ALD) is 72-76MHZ. The frequency is prone to distortion, tends to be tinny and has a very short range. An IEM unit is capable of ascertaining full stereo capability when done correctly and tends to work between 500ish to mid 600 MHZ. So there is a substantial quality difference, just to be clear.
And, no, they are not in fact illegal. Same as recording a concert with microphones; one would have to prove intent to profit in order to prosecute. The circumstances when tapers get busted is when they sell their recordings.
Also do not waste your money on an ALD receiver, there are so many better options I believe.