Agreed. "Free" power just means someone else is paying for it. I find it distasteful, and not because I'm jealous--I manage machine rooms and could easily increase my organization's electric bill for my own profit. Or I could just steal money from the till--it's all the same at the end of the day.
It's really all about how you define "free." Obviously stealing power from someone else isn't "free" power.
But what about power that is sold at a flat rate? Some firms have arrangements where they pay $X for Y Kwh per month. If the firm actually uses Z Kwh per month, and Z < Y, then the firm has Y-Z "free" excess capacity to use each month. This type of arrangement isn't the norm, but it's common enough, and if the owner of the firm decides to use that "free" excess capacity, then it is indeed "free" to them (of course, that assumes that the firm has absolutely no need for the free excess capacity, and could not pay less for a lower amount of capacity).
Maybe not the best example, but when some people say "free," they may mean power that they've already paid for, but aren't using. Sure, technically it isn't free, but relevant to this topic it may as well be, as there is no marginal electricity cost for these people to run an Antminer S7, provided they have the excess paid-for capacity.
If they state it that way instead of I have free electricity, then people would understand. For example, Ivanpah solar thermal plant may generate electricity in excess of what they can sell during a sunny day, plant manager may allow employees to use the excess but it only works during the day just like solar panels as some has mentioned. At night the plant is using power from the grid.