It's not their obligation to track down cheaper or free sources. That's silly. After all, the ACM isn't going to waste time keeping track of the various sources of every paper they publish. It's not their job.
If someone is motivated by the fee to find another source, they'll do so. Maybe they'll ask their colleagues. Maybe they'll search the web. Maybe they'll take the time to visit a university library and track down a print copy.
But if they don't care about the price, they'll just do the easiest thing and pay the ACM in order to save time.
It's not the ACM's job to do the legwork. If you want to save the money, you have to do the work yourself.
If someone is motivated by the fee to find another source, they'll do so. Maybe they'll ask their colleagues. Maybe they'll search the web. Maybe they'll take the time to visit a university library and track down a print copy.
But if they don't care about the price, they'll just do the easiest thing and pay the ACM in order to save time.
It's not the ACM's job to do the legwork. If you want to save the money, you have to do the work yourself.