Very often, the parkway or land near curbside is public property and not personal property. You put your garbage cans there, you've "discarded" their contents.
I believe the same applies to alleyways.
(IANAL and I don't know the specific legal arguments.)
Yeah I've heard this but the trash can wasn't discarded, I could see how a trash bag alone maybe since all of it is going to the trash but an actual trash can with a closed lid? What is the rule around mailbox? Would opening the mailbox door be considered trans-passing? Wouldn't that apply the same to a trash can?
The mailbox is/becomes USPS (U.S. Postal Service) domain (I don't know about "property"). Which is why you'll never find handbills placed in them -- it's against the law, and the postal service will fine and/or prosecute companies that violate this.
Again, I don't know all the arguments, but you can, for example, leave your bicycle on public property without losing your property rights to it. (For that matter, you can park your car on a public street.)
I believe the convention is that, when you've placed your trash for pickup, you've also discarded your property rights. Where the line is, though, for example with regard to "placement"? Any lawyers want to chime in?
P.S. Or, I/we could wade through the other comments in this now much larger thread.
I believe the same applies to alleyways.
(IANAL and I don't know the specific legal arguments.)