First, let's not call these implementations I think that's partially where this confusion that these JARs are more that just the spec JARs.

Licensing is a decent argument. I did some digging and the javax.transaction:jta artifact is licensed under CDDL.

Other than licensing, none of these apply to JTA. Yes, the "availability" angle is certainly valid as a spec is going through approval and even for a short time afterwards. But JTA 1.1 was released in 2006