I guess paying employees, building towers, capital investments, etc. don't count
Sometimes the easiest explanations leave a lot of things out
Saying that offering the service costs them nothing is a little naive
No, it is not costing them anything.
It is true, that you need the techies, the towers, the capital investments, BUT you did not invest in all of that for the SMS network. These posts are paid for by the voice and data services for what they are build. To have a voice or data service, you have to connect to the tower. From there you go either via directional radio to the next tower or thru fiber to the backbone. Whatever, that is the infrastructurer the providers invested in and this infrastructure is being used and paid for for these services and there customers.
One of the technical issues is, to connect a handset to the system, for that it sends a individual code for the handset and the SIM and then is permitted to go online. So far, so good. The authorisation as well as the tracking and handovers to the next tower (in case the handset is moving) are done in a small carrier your device is permanently sending and receiving. Over this carrier wave is all kind of information send. Of course, your personal code, to be allowed on the network, the speed you are travelling, your location ect pp. This signal is not to be confused with the large and powerful signal that is being send when you actually speak or transmit data. But it is always on and because it operates on a much lower frequency then the actual mobile network, you can often see a signalbar and send or receive messages but can not connect with voice or data.
This carrier wave is used to send and receive SMS and the slow and small band nature of this carrier is also the reason, why a SMS is limited to 160byte chunks. In the early days of GSM, SMS where free, they where a side product nobody cared about as ist did not need any extra frequency oder other technical stuff then that, what was already there (and was already paid for)
Similar to the old days when you just called a number and let it ring, once for "I'll be coming soon" twice for " get dinner ready" three times "will be late" ect. At no cost, the line was there and paid for. SMS do not cost the provider anything extra on top of there needed, installed and paid for infrastructure.
Only when Nokia and Ericson in the last century introduced this "new" message feature to there phones (if you could call those bricks then phones) it took off. The telecoms quickly saw an easy money making oportunity and started charging for the service. Soon, a big chunk of income came from these services. Understandable that they do not want to loose this free money.
As a matter of fact, the most expensive equipement they have to install is actually the metering facility. Before you could simply send and receive SMS, in order to charge, there had to be tech installed, so they can control what you send and how much. SMS are not encryted and can be read by anyone that has youR network access code, easely found out standing close (within a mile) to your handset. If you know the (lately often changing) access codes to the message centers, you can send SMS for free by bypassing them, you can even use this carrier wave for internet access, if you are content with 8baud speed.........