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

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.