Lack of autocommit would be bad for performance at scale, since it would add latency to every single query. And the MVCC implications are non-trivial, especially for interactive queries (human taking their time typing) while using REPEATABLE READ isolation or stronger... every interactive query would effectively disrupt purge/vacuum until the user commits. And as the sibling comment noted, that would be quite harmful if the user completely forgets to commit, which is common.

In any case, that's a subjective opinion on database design, not a bug. Anyway it's fairly tangential to the client library bug described up-thread.

Autocommit mode is pretty handy for ad-hoc queries at least. You wouldn't want to have to remember to close the transaction since keeping a transaction open is often really bad for the DB