Basic economy tickets aren’t a ‘new way to offer cheaper fares’. They are new restrictions on the cheapest fares, a way of making the product worse so that some passengers will pay more.
Now that United, Delta, and American all offer Basic Economy fares (no advance seat assignment, no cancel/changes, no upgrades or extra legroom seats, and in United’s case no full-sized carry on bag) it’s only been a matter of time before one of the carriers tried booking award tickets into Basic Economy.
I admit to being surprised that it’s happened so quickly, since it’s not in the best interests of the airline – it’s a bad business decision – but I’m not surprised that it appears to be Delta going first.