Again, their is no need for a "right to privacy", we inherently posses all and any rights not explicitly forbidden in the law or granted to the various Governments to which we are subject. The Constitution is an explicit listing of powers or rights that the Federal Government and it's agents may posses. They are not granted any more authority than is explicitly listed in the Constitution and whatever other laws we've passed. You could argue that some of this might be covered under the questionable patriot law clause, but that is more and more tenous by the day.
Intent and method matters when you are talking about communications, otherwise there would be nothing illegal about wiretapping any phone conversation. You are essentially arguing that just because you had a conversation using a STU in a fortified bunker in an old mineshaft that there is nothing wrong as a private citizen tapping that line, cracking the encryption, and doing whatever they want with it. Or say a voyeur installing their own airport type scanner in a public walkway of some sort disguised as something else, I mean nobody is bothering to even encrypt the radiation they are emitting or reflecting. Just because they wouldn't expect anyone to be able to see or detect that, or even know it's possible, their can't possibly be anything wrong.
And speaking of radiation. I would wager that these towers are also in violation of any number of FCC Regulations on the use of radio frequencies. The frequencies that are used by cell phones are very lucrative and are leased on an exclusive basis. These towers are very likely using bandwidth that is already leased to some other entity. Remember they aren't just listening they have to be broadcasting or the cellphones would never actually establish a connection with them.
So far as the DMCA goes, it is a law, as far as I know it applies to everyone unless they are explicitly exempted. According to the DMCA it doesn't matter how weak the encryption you are using is. Deliberately dycrypting something which you have not been explicitly authorized for is a violaiton and hacking of some sort or another.
The mailbox is exactly like radio transmissions it is just visible from a longer range and using a naturally visible frequency. When I put something in the mailbox it is continuously available to anyone who has the gumption to open the box and open the envelope. When you put a letter in an envelope and seal it, it is then obvious to anyone that it is private and not intended for anyone to read but the receipient listed on the envelope, doubly so if it's one of those 'security' envelopes. But the actual physical protection on that letter is still actually incredibly weak, my two year old frequently opens my mail for me. The same should be logically true for encrypted radio transmissions. Just because it is trivial to break does not justify deliberately doing so. And by virtue of those transmissions being in specifically leased frequency ranges they have an expected receipient, which is whoever the wireless provider is that leased the band and signed the contract with the end user. If the mailman accidentally, or even deliberately delivers my mail to your mailbox you are not legally allowed to just open it and read my correspondence.
As a private citizen I do not need an explicit recitation of rights, the only things I need explicitly stated are things that I may not do. The Government and its many agencies and agents does require an explicit grant of rights in the law to perform their duties. That is pretty much the entire point of our Constitution.