I'm about to start looking for flying pigs because this is the thing I was most sure in the world WOULD NEVER HAPPEN.
Some of this has to--must be caused by the fallout of--the Sony hack in December. There were hints then that some talks had happened, but I think it wasn't until Sony was materially damaged by the hacking that it made this possible. Between that and the dismal "failure" of Amazing Spider-Man 2 (for definitions of failure that mean "did not make a billion dollars"), Sony needed some of that sweet, sweet Disney money bad. They're apparently keeping the rights to Spider-Man himself, so there's still every chance for them to completely run the character into the ground before the rights revert to Marvel/Disney, as I originally suspected would be the only way Marvel would get Spidey back.
Aside from being amazed that this is happening, I'm also worried? I think the greatest strength the MCU has had in making it a marketing monster is that it has a plan. Good or bad individual movies can come and go, but there is still the universe, and there is still interconnectedness driving the success (financially, even if it's frustrating artistically for some of the players involved; directors, especially, seem annoyed). Unless they have worked Spider-Man into that plan on the hope that this would happen, not having him integrated into that plan is potentially threatening to the success of the whole (although, granted, not as much as Ant-Man seem to be).