If AT&T made a Text 81841 For Guns and created an anonymous infrastructure for arms dealers to sell guns to gangsters, yes, they would be in trouble.
Here's another example - I'm sure that some people have used Reddit for illegal transactions, but no one is going after Reddit for facilitating drug trafficking because it's a small part of their customer base.
In contrast, the Silk Road was wholly dedicated to selling illegal goods. That's why it was created, and that's why it made money.