It's because of the fact on free services, if you put in files beyond a certain size (200MB usually) they WILL NOT allow you to download it unless you own the file. Some (such as mediafire) put additional restrictions, including blocking uploading and downloading multisplit archival files.
Or simply put, if I upload it to mine, you will be denied access to them, even if I provide a link to them, and even if it worked, the space is currently in use.
You can of course pay to have these restrictions lifted, but it requires you to, well pay for them. If you are a premium subscriber, you can provide a dropbox facility, and this means you just need to tell me where to drop it off to.
Since I run on operational security, I require if you provide me an account (so I pretend to be you) that it's not one with your details attached, as I am not interested in identity theft. I'd need the username/password/service PMed though.