"Treason is obviously not allowed."
The -lu' suffix sort of turns things around. Who is doing the allowing or not allowing? Not the treason. Some generic unspecified person. In English we might say, "One obviously does not allow treason." That wording seems a little odd in English, but perhaps can give better insight into the word order of the Klingon.
Good thinking, though. If there were a verb be not allowed then 'urmang would come last. But because the verb chaw' means allow, treason still is its object, the thing that isn't allowed, so it comes first.
'urmang chaw'be' SoSwI' = My mother doesn't allow treason.
The trick with -lu' is that you can say something is done while taking all emphasis off who does it, just focusing on the thing done.
'urmang chaw'lu'be' Some unspecified and not important to the sentence entity doesn't allow treason.
Klingon isn't even that weird. English, lacking a -lu', unless you count one, has to resort to inverting the sentences and saying "Treason isn't allowed." But that's English's problem.