CPButton fires actions and draws alternateImage when disabled

Created a button with the following code and slapped it in a view:

        _forwardButton = [[CPButton alloc] initWithFrame:CGRectMake( 22, 0, 22, 20 )];
        [_forwardButton setBordered:NO];
        [_forwardButton setImage:__forwardButton];
        [_forwardButton setAlternateImage:__forwardButtonHighlighted];
        [_forwardButton setImagePosition:CPImageOnly];
        [_forwardButton setEnabled:NO];
        [_forwardButton setTarget:self];
        [_forwardButton setAction:@selector(_forwardClicked:)];
        [_forwardButton sendActionOn:CPLeftMouseUpMask];

Notice that the button is disabled with setEnabled:NO. When I fire up my app, the button image appears to have been dimmed (looks like 0.5 alpha) but it is still clickable (it draws the alternateImage on mouse down) and fires its configured action on mouse up.