It's an incredibly common use case to forward text messages from one incoming line to another number that is controlled by your company, and it's impossible to tell at the point of receipt (when it's still in Twilio's network) whether it's spam or not before forwarding it on.

It's the same exact thing that you might do if you have a support@example.com email address; it probably sends incoming emails to a number of different inboxes, even if those incoming emails are spam (and even if there actually is a spam system in place that might block some percentage of incoming messages, and Twilio doesn't even provide that.)