If you're going to hold people responsible for meeting a metric, you need to ensure they have a way of measuring the thing in question. To do otherwise is unreasonable by definition. If Twilio's definition of "sending spam" matched the OP's, where it's about sending messages the user did not agree to receive, then that would be reasonable. But instead it's something more intangible.

And like you say, they do need to prevent their system being used for sending spam. But if they're not going to provide a way for honest parties to avoid sending spam, they need to be reasonable about how they react to spam reports.