It's because the expressions separated by commas are evaluated before the whole comma-separated tuple (which is an "expression list" in the terminology of the Python grammar). So when you do foo_bar_tuple=="foo", "bar", that is interpreted as (foo_bar_tuple=="foo"), "bar". This behavior is described in the documentation.
You can see this if you just write such an expression by itself:
>>> 1, 2 == 1, 2 # interpreted as "1, (2==1), 2"
(1, False, 2)
The SyntaxError for the unparenthesized tuple is because an unparenthesized tuple is not an "atom" in the Python grammar, which means it's not valid as the sole content of an if condition. (You can verify this for yourself by tracing around the grammar.)
