Using the copy command without parameters may seem strange if you're used to more complex languages, or necessary if you're implementing an unusual interface.
The clipboard belongs to the user, and an application should preserve anything the user has explicitly placed in it. Using it for any other purpose risks overwriting data the user had copied that may be important to them, perhaps even for use in other applications unrelated to yours.
In most applications, copy works on an active text selection within an editable field. In that most common scenario, LiveCode makes it as simple as can be to do what you want while leaving the user in control:
...is all you need, provided the user has selected a range of text to copy.
It's very rare that copy is allowed for uneditable text in a list field. Offhand, I can't think of any examples where I've seen that. It may indeed be a very useful thing to provide in your specific application, but in such an unusual circumstance we can expect to need unusual syntax to support it.
Thankfully, as the others here have shown, LiveCode is flexible enough that it requires at most just a few extra tokens or a second line of code to do even uncommon things like this.