Adding missing FROM clause - thanks or no thanks?

I recently had a problem where a bunch of postgres backends were taking up
huge amounts of CPU time. I found a bunch of log messages like this:

NOTICE: adding missing FROM-clause entry for table "b"

Which I eventually tracked to an improperly constructed query that reduced
to something like this:

select a1, a2 FROM a WHERE a1 = '...' AND b.b1 = '...';

Table b is pretty big and the conditions were such that it was (apparently)
doing a join of every row in a with every possible row of b. That's
certainly now what I wanted.

Anyway, I fixed the query construction bug, but I have a question: is this
"adding missing FROM-clause entry" behavior part of the SQL standard(s) or a
"feature" of Postgres? Prior to seeing this, I'd have expected a query like
the above to cause a fatal error in the parsing stage. Put another way, can
someone give me an example of when this behavior is useful or desirable?

-John