Like I said, just installing steam games onto a few computers in the house is enough to breach the 250GB limit, and that wasn't even close to all the games that we have.
There are plenty of other legitimate reasons as well. Take online off-site backups for example. People who need to do an online backups of their photo's all the time. Photographers for example. Shoot, teens who feel they need to upload every picture they've ever taken to Myspace AND Facebook.
Or take my brother, who is uploading very large HD game demo movies just about every day, on top of the ones that he downloads to compare his work to.
And the biggest thing it seems you are overlooking is that it isn't 250GB per person. Some people have large families all within the same household. Now have all these people all doing their own independent thing, and it is easy to see how it is legitimately possible to break the limits. AND, all it takes is once if your phone number isn't up to date.