This is something I've thought about.
Simply because my employers/managers have expressed to me that they work a lot of overtime hours without charging for them, because they're over the budget for clients.
(Well if I'm being completely honest, the clients asked for more revisions and work, but the managers don't want to charge, because they will lose the “bid” and possible relationship with client. I have the impression they don't have much of a choice because if they set up boundaries, another company can simply undercut them and hoard all the clients).