Eh, when was Forza the "main" sponsor for Le Mans, I'd like to see that article. I've never once heard about that. I know they sponsored a V8 Super Car Team at Bathurst, but the whole Le Man's, never heard of this. I'll do some research. Be back with details.
Wrong, if they are the major sponsor if they pull out then the race may not happen at all.
Edit:
"“Forza Motorsport” was there, not only represented by an amazing driver, but also as an official sponsor for the world’s premiere sports car racing event.
Read more at http://news.xbox.com/2014/06/19/games-forza-represents-at-le-mans/#3UcTWDAuM5CSomZe.99"
Major sponsor no, official sponsor yes, just like a myriad of other official sponsors like Gran Turismo always is an official sponsor for ALMS and Le Mans. Major Sponsor means you sponsor the event itself in a big way. Gran Turismo isn't a major sponsor for Le Mans but their logos are on every car in the races, hmm might be more to this than I thought. Will do more digging. That quote came straight from MS Xbox page article on Le Mans from a few years back.
Their billboards were all over the track, they obviously paid a LOT of money to do that, which was your argument why Sony would want to lock it up.