Clear Picklist, Why can't this be used in custom button onClick?

Clear Picklist is a client side function.
Why can it not be used in a custom client side button?

I get this message.
Error at line 73 : 'hide | show | enable | disable | clear ' can be used only in 'on load' and 'on user input' actions

I would think that onClick on a stateless form custom button would be considered a 'on user input' action.

tt