Let me clarify a few points here.
1. Twilio is not taking this action because they hate weed or want to read anybody's messages. It's being forced upon them by the actual telecom providers that send the signals (e.g. T-Mobile). Twilio does not own the communications service, so they have to abide by the policies of the carriers that place the messages in people's phones.
2. The SMS network, like all communications networks these days, is international, and not every country has the same approach to free speech. In particular, there are certain countries where specific types of message content is illegal. This is outside of the control of any carrier, Twilio, or OwnerRez, and, since by their nature cellphones are portable, can have all sorts of difficult to handle effects.
3. SMS messages were originally a tech hack used by cellphone service technicians. They weren't designed in the first place as a proper service to be sold. As such, much like email, a lot of things weren't thought through in the way we'd do it today. In particular, SMS messages are simply text in-the-clear - they're not encrypted themselves in any way. Now, modern digital cellphones do encrypt their entire communications stream between your phone and the tower, so people can't just listen in on your conversations with a radio scanner the way they could years ago - but, inside the carrier network, the SMS messages can be read by any system with authorized internal access to that network. The same is true of email, which is why spam filters work. Therefore, yes, the carriers do have the technical capability to read and filter messages, and government authorities have the ability to regulate and require such.
There is no technological capability, by OwnerRez or anyone else, to securely encrypt conventional SMS text messages end-to-end such that they can only be read by the sender and recipient and not by anyone in the middle. SMS messages just don't work like that natively in their standard form.
There are, of course, various applications that run on top of the SMS message system that can encrypt text messages, and then send the encrypted result. That way, yes, nobody in the middle including the carrier can read the content. But to use these systems, both the sender and the recipient must be using the same encryption application, and there is currently no established standard or guarantee that any particular phone number is using the encryption app you've chosen.
In the vacation rental context where you want the maximum potential guests, using such a system would be like requiring all guests to communicate with you only via WhatsApp - sure, lots of people use it, but lots of people don't, some refuse, some aren't allowed to for various reasons, etc. The result of such a policy would be to greatly limit your pool of potential guests.
The virtue of conventional SMS text messaging, as with standard email, is that you can be pretty much guaranteed that every potential guest can use those systems.
