I actually spoke to a SkyDrive team member about this one. The root problem is that once you have a truly shared folder, it takes up both people's quota (it has to, otherwise you could link a lot of free accounts to get extra space). The way it is implemented now makes it an explicit operation to move something to your skydrive, so you know the quota implications up-front. That said, they are trying to figure out a better solution for this.

Ironically (or not), this is the reason I quit Dropbox, because they do not warn you that a shared folder will ruin your quota. Someone shared 50GB of photos, it started syncing, maxed the quota, and resulted in some important documents never making it to the cloud.

Neither is an ideal solution. Nobody's solved this problem yet.

Edit: The other reason was security; auto-sync is an excellent vector for malware to enter the system. Get into one person's skydrive, infect their shared folders, rinse and repeat.