When I first created Tiny Core, It was a conscience decision not to include such. Part of my tiny and nimble philosophy.
I am sure that you are aware that there is two levels of copy and paste. There is the selection (copy) and then middle click on a target (paste). Then there is the clipboard where after a selection, either a menu or control key is used, to copy to clipboard. Then the target typically does similar with menu or control key to paste.
Adding all this to many GUIs would certainly bulk up the system. Especially adding menus and control keys. And for the middle click method my thought was "How likely would a new user to Linux know about the mouse click method?" Offering the mouse click method would likely lead to requests for the full menu and control keys. I resisted the notion. Perhaps the mouse click method would be a compromise, if it is even supported in FLTK 1.1.10. I see FLTK 1.3 improves on handling of such with more fully implemented methods.