They're a web host - not a backup service. There are backup services out there that won't let you run a web site from their hard drive space, even though you're paying for it.
You can't go to an all-you-can-eat buffet and dump all of the food trays into the trash can. If they'll let you have as much as you want, as long as you eat it while you're there, why shouldn't you just be able to do whatever you want with it?
All hosts are going to have content restrictions of some sort, but not all hosts give you backup space like Dreamhost where those restrictions are lifted (except for illegal content, of course). This is something they could have easily sold separately or charged for as an add-on.
Anyone that requires more storage than CPU usage. If your site is harder on space or bandwidth than it is on the CPU, shared is often fine. If it's a CPU hog, then you pretty much get forced onto your own server at some point.
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