Makes sense to me. Terminal maintenance, electricity all these cost add up. So tip is due. After all it is customers' responsibility to keep track all expenses business incur during course of its operations. Paying just listed price and taxes is so "heartless".
I keep forgetting that listed prices, at least in the US, can be before tax.
By contrast, in the EU prices are so transparent that whenever there is a sale (in person or online), the new price has to be accompanied with the lowest price from the last 30 days - so that the customer could check how big that sale really is.
