“Why did the police officer smell?”/“Because he was on duty.”

A police officer riddle is:
 
Q: Why did the police officer smell?
A: Because he was on duty.
 
The joke has been cited in print since at least 2006.
     
 
Only Simchas
P.S. Why did the policeman smell?
Wednesday Dec 6th 2006
     
BimB-FUNNY
Wednesday, November 11, 2009
Short Funny Jokes
(...)
cop
why did the police officer stink? Because he was on duty!!!!
 
Twitter
Patrick Soria
‏@Stryke02
@Cerrone23 why did the police officer smell??  cuz he was on duty!
4:29 PM - 9 Dec 2009
 
Twitter
Desiree G
‏@desiree85
Q: why did the police officer smell? A: because he was on duty.
1:17 AM - 20 Dec 2009
 
Reddit
Why did the police officer smell?
submitted April 20, 2013 by hsy1234
Because he was on duty.
 
Google Groups: GayToday
BEST. SHORT JOKES. EVER.
mokemoke
6/17/13
How do you make a tissue dance? You put a little boogie in it.
Why did the policeman smell bad? He was on duty.
 
Google Groups: Corny joke of the day by Karan
8/5/13
Ling Ling Ritter
8/6/13
Enough of the football jokes.
Why did the policeman smell bad?
He was on duty.  :p
 
Twitter
Laffy Taffy Jokes
‏@Laffy_TaffyJoke
Why did the police officer smell?
Because he was on duty.
5:42 PM - 14 Jun 2014
 
Reddit—3amjokes
Why did the police officer smell? self.3amjokes
Submitted July 14, 2014 by klou12
Because he was on duty!