Because the money it doesn't throw away for nothing more than covers the expense. Sales aren't some abstract thing that can't be quantified. Site gets no traf

Because the money it doesn't throw away for nothing more than covers the expense. Sales aren't some abstract thing that can't be quantified. Site gets no traffic -> Spend $X on google or facebook pointing traffic to site -> Y Amount of traffic -> $Z amount of sales. You now know how much revenue $X worth of ads buys you. You also know how much each amount of revenue is worth to you in profit. As long as spending that $X makes profit, you're gonna do it. No matter what crap is mixed in by the ad company. If something you want is bundled with something you don't want and is priced such that you're willing to buy the thing you want at that price, you don't turn it down. You buy it and ignore the thing you don't want.