This isn't even an issue just with 'professional interpreters'. I participate in a re-enactment group that has a high level of authenticity and research. As a group, we charge for the majority of events to hire us. Yep, even though we are hobbyists. Our group still has costs that need to be covered (petrol and insurance, to name but a few!) and we are still, for the most part, highly skilled and knowledgeable individuals with high-quality costumes and equipment, not just Jo(sephine) Bloggs off the street plonked into a synthetic pretty princess gown from Ebay.
That said, one thing I think Isabella has missed in her (quite justified) rant against being asked to work for free is that there are sometimes reasonable reasons why you might choose to do this. (Note: choose. People assuming you will work for free, particularly when emailing you at your business email, are just obnoxious.)
So here, a handy dandy guide to how not to be an arse when attempting to book historical interpreters (professional or otherwise).