Signs in states which say "Only Trash Litters"

In many states I can see signs posted which state "Only Trash Litters" which I certainly have no problem understanding and which appear to be correct usage to me. "Trash" can be singular or plural so I have no problem about this. If the s is dropped on "litters" would both be correct? Or is one use correct?

"Trash" in this instance, besides being a play on words, in one of its meanings may be construed as a collective noun. Collective nouns may use singular or plural verbs.

Without the use of the singular here, the double meaning wouldn't work. It would be reduced to a single-entendre.

As others have stated, most people have no problem whatsoever understanding that a sign that reads "Only trash litters" is correct and refers to a person that would litter is trash or trashy. Litters is the singular form to use. The gist is easy.