Apple has been baiting its retail employees to form unions given the relatively shabby treatment the company has given them. They're not treated as real Apple employees with value but rather retail drones, so I'm not too surprised to see this development. In fact, I predicted that such a thing would happen after reading reports about Apple's poor practices in this area.
As for self-checkout killing those same jobs, I don't think that's the intention. Self-checkout is being introduced because there are too few employees dealing with too many customers, so for smaller items the customer gains the ability to pay for an item and walk out without having to seek out an employee, freeing up the employees to handle the bigger ticket items more exclusively.