That's because they know they should just tell you that they don't need to pay you less–you're a salaried employee. Salaried employees get paid a fixed salary to get a job done. The part that employers enjoy in the arrangement is you being able to work more hours to get something done and your compensation remains a fixed cost. What they don't like to talk about is the other side of the coin: It doesn't matter how much time it doesn't take either. So accordingly you should, in theory, be able to work less days if you get your job done in those days.

But the reality is that many managers and employers only support the part that benefits them. Even many of the ones that are supportive of someone taking a Friday off every other week feel that they're doing you a favor or giving you a "perk". At the end of the day your manager has to support it (and if necessary continue believing they're doing you a favor) because they can find a way to shit-can you if you start pushing boundaries they're not comfortable with (in the US).