I recently had a year of Sky TV for half price - all that means is they made less profit from me for a year than they made from a full paying customer.
I would say that the "full paying customer" is subsidising Sky's profit margin, (generally fixed and subject to scrutiny), favouring the hagglers.
Nobody is subsidising anybody else. That simply isn't the way the business model of Sky works on the TV side of things.
Sky's costs are virtually "fixed" in terms of providing their TV services. Once Sky hit a break even figure, most after that figure is profit. If Sky don't hit their break even figure on TV supply costs, they would lose money. So assuming Sky hit the break even figure, any customer paying less than another customer is simply contributing less to Sky's bottom line than the customer paying more.
You could even argue that "hagglers" are protecting full paying customers from having to pay more, with a very simplistic illustration of how that could be:-
Sky fixed costs = £1.00
Sky full paying customer charge = £0.75
Therefore Sky loses £0.25 and has to increase charge by £0.25 so the full paying customer has to pay more.
Alternative
Sky fixed costs = £1.00
Sky full paying customer charge = £0.75
Sky customer that haggled charge = £0.50
Therefore Sky then makes a profit, and the full paying customer doesn't get a price increase.
So it could be argued that "hagglers" are actually subsidising "full paying customers" by protecting them from having to pay even more.
Full paying customers should thank those that haggle IMO, as us hagglers are keeping their prices down.