Did they ever ask? You can ask the SEC for a "no-action letter", where they will officially tell you if what you want to do is OK. Here are all no-action letters.[1]

Here's one for Paxos.[2] They asked for permission to run a small-scale blockchain-based clearing operation without registered as a clearing house. No more than 7 institutional customers, a limited list of stocks, and a time limit. They got it.

Somehow, Coinbase doesn't seem to be on the list there. It's a public record.

[1] https://www.sec.gov/divisions/marketreg/mr-noaction

[2] https://www.sec.gov/divisions/marketreg/mr-noaction/2020/fin...