There's a very important reason that YES doesn't want to do this, at least right now. Cable networks pay YES a little less than $7 per month for EVERY basic cable subscriber in their markets. Whether you watch or don't, the cable company pays that fee. So yes is getting paid for so many more people than actually watch the programming. In order to make that up with a direct subscription, they would likely have to charge a lot more than $7 per month to app users. So, for now, the cable operators have a huge amount of leverage over YES. As cord cutting continues, that will change, but for now, that's a lot of cheese coming YES' way.
Discovery can get away with it because they are too important to cable operators because of the broad appeal and sheer number of stations.
HBO and the pay channels can do it because they don't get paid for every basic cable subscriber, just the ones that actively choose HBO.