A little research leads me to believe that this varies greatly. Some payment processors keep everything (and get merchants to sign up by persuading them that it's great for their customers and "there is no cost to you"), others appear to pass most of the additional FX margin on to the merchant (see the section "how we make you money" near the bottom of this page for an example) -- although we can assume that this is not an act of generosity: the payment processor will benefit from the higher amounts being charged and may well be charging more for terminals/software etc. in the first place.

I have not yet seen any reports of businesses paying a bonus or commission to staff who 'sell' DCC to customers, so for the time being will continue to assume that individual employees are not knowingly trying to scam me.