It can be. For years, there was a section of the Southwest Freeway access road in Houston that was posted at 35MPH when, according to all legal requirements, it should have been posted at 45MPH. If you knew this and got a ticket, you could just tell that to the judge and the ticket would be dismissed. The posted speed was unenforceable.
The Houston Police Department continued to write a large volume of 10MPH-or-less-over tickets along that stretch for years even though they knew they shouldn't. Personally, I'm happy to call that a speed trap.
I understand what you're saying, but ............................. if it's posted (even if someone thinks or says it should be higher) and the posting remains, why would that be a "trap"?