If an airline makes a fare ineligible for any FF benefits, they are effectively saying "if you are in the market for this kind of fare, we do not care about your loyalty". Which is entirely their prerogative to say. Travellers can then be free to choose the lowest fare without the psychological encumbrance of worrying about mileage accrual.

UA has made certain business decisions about who they want to target with their FF program and AC has made different ones. Remember AC dropped accrual on the lowest fares domestically to 25% quite some time ago. Going from 25% to 0% isn't much of a leap.