The whole point of ATC is that everyone in the area is able to figure out what is going on by listening in.
What/how is making listening in on that illegal ok?
[delayed]
> Totally legal everywhere (except a few rare exceptions, like the UK).
And (after a cursory search) Spain, Portugal, Germany, Austria, maybe France... In other words, legal in some places, illegal in others.
> Text-based ATC communications is done over a different system called CPDLC (FANS-1/A)
ACARS is both an application and a (legacy) lower layer suite of protocols supporting it, but modern ACARS versions and CPDLC can use the same underlying digital channel, as far as I understand (i.e. VDL Mode 2).
As a result, many of these tracking sites can capture both, as well as presumably "legacy/analog" ACARS.
Not sure about the details, but I suspect it's more a consequence of laws on telecommunication privacy from analog days being very generic than a specific intention to make ATC listening illegal. Opt-out vs. opt-in by frequency and/or purpose, essentially.
That said, it's supposedly still being very much enforced against e.g. planespotters at airshows in some places β no idea what the point of that is.
Baffling to get downvoted for a suspicion about the origins of the status quo, as if that were an endorsement of it.
But so were analog mobile phones and pagers, and in some countries, even receiving unencrypted voice ATC radio isn't legal.