I wonder if that's a crackdown in cannabis, or if the cannabis industry is forced to use sketchy carriers.

They couldn't find any telcos willing to take cash. Luckily some fellas were willing to accept Steam gift cards.

That sounds like a system working perfectly correctly. What is the issue?

The issue is that legit, licensed businesses -- calling each other to conduct lawful business (not spam, calling to discuss pricing on existing deals) and T-Mobile blocking the calls. So, for example, a Verizon client is calling a State enforcement officer who's device is T-Mobile -- now the licensee call cannot go through to communicate with their enforcement officer -- which can lead to bad things -- such as LEO at your door with dogs and guns.

Some of the only spam messages I receive are ads for cannabis shops. I've never even touched a plant.

Not just T-Mobile; Twilio is also (selectively) hostile to cannabis. T-Mobile blocks (or tags as spam) 100s of legitimate, licensed businesses. From the FCC: "Universal service is the principle that all Americans should have access to communications services" -- and Twilio, T-Mobile (and other providers) make selective choices which are against that policy.