Unsure why episodes wouldn't be available on iTunes. You'd have to pay for them individually, so CBS would still be making their money - in fact they'd make more. A regular HD episode of Fear the Walking Dead is available on iTunes for $2.99. Since the full season of Discovery will consist of 13 episodes and on the working assumption it'd follow the same pricing model as Fear, that's $39 for all episodes in season one. iTunes also has the "season pass" alternative where you pay in full - generally a couple of bucks less than if you were buying episodes on their own.
I don't know if regular broadcast network shows offer the season pass or if that's just limited to specific cable shows. Perhaps Discovery will be available on iTunes after the season has been made fully available on All Access. Marvel's Netflix-only shows come to iTunes eventually - usually not long after the show's been out on Netflix and the word comes down they're moving ahead with a new season.
It's more money for CBS, so I don't see why not. Though Apple probably gets a percentage, I'm not sure how much.