The inactivity policy is spelled out in the
Terms of Service (which you said you read and agreed to when signing up) as is the termination/deletion policy. Essentially, it takes 44 days for an account to go away: 30 days of inactivity, plus a further 14 days during which the suspension for inactivity can be removed and the account put back into service. After that time, the account is terminated and the files deleted from the server and removed from backup. You are also informed that the only reliable backup is the one you create yourself locally (or, if you can't do it locally, you could use an online file storage system like Dropbox).
Essentially, logging in is your "bill" for the Free Hosting service, and it's the only way to determine which accounts are still active/wanted. (You can also purchase Prime or Illuminated membership, which doesn't require a login; the payment itself is sufficient indication that you want to keep the account active. And if you know you're going to be away for a while, you can avoid the login by purchasing a short-term Prime upgrade to cover the period.) Data storage might be on the cheap side these days, but it is not free (disk space costs real money, and backing that data up costs real time) and maintaining all of the data that people just walk away from would be both expensive and impractical. I know—you "only had a few files". So does everybody, ever. They add up to a lot when you put them all together.