Same thing with Unmetered.
Well, no...not the same thing with unmetered. I think you missed my point. Perhaps another example will help.
Ex.
I set my Xen VPS servers up on a dedicated server with 100 mbps dedicated connection. That 100mbps pipe is all mine, not shared with other servers. To make things really simple, we'll say it has 8GB RAM, and after overhead, maybe 900GB of hard drive free space.
For simplicity sake, we're also going to assume that I only have one VPS plan, so to avoid overselling, I'm going to want to split the resources up equally.
Plan:
100GB disk space
1GB RAM
?? Bandwidth
Hmmmm...how would I divide the bandwidth? Well, the 100mbps pipe will provide around 30.5 Terabytes of data incoming and outgoing, in a frictionless, theoretically world. So to be safe, we'll knock it down to 25 TB incoming, 25 TB outgoing. So split between the 8 clients, we can offer each client 6.25TB of bandwidth (transfer, really) and still be safe.
So everyone on my server is happy. They're happily hosting websites and transferring files, but let's say 4 of the 8 clients only use 1TB of transfer on average per month. A good chunk of the available bandwidth each month would be going completely unused on the server.
I'm paying for the dedicated pipe every month regardless of how much it gets utilized, so why not let the clients use it? Why not offer the clients 100mbps unmetered bandwidth on each VPS, noting that the connection is split between 8 accounts? There's your cap. Or even call it unmetered with a set minimum amount, i.e. 6TB burstable to 100mbps beyond that? Several of the accounts might then quite easily be able to go beyond the 6.25TB, and utilize the available bandwidth that the other accounts aren't using.
Perhaps one of the "no unmetered at any time...EVER!" folk can come up with a set bandwidth plan for the above scenario, that would allow the full pipe to be utilized? Well...no, don't bother, because you can't, without simply overselling.
As I'm hoping you can clearly see now, it's perfectly acceptable to offer unmetered connections (with a set cap) at the dedicated and VPS server levels. You could technically do it with shared and reseller hosting as well, but there are just WAY too many factors, the biggest being the amount of clients. What are you going to say, unmetered 100mbps connection shared between 300 people? ROFL! Who the hell knows how much transfer you'll actually be able to utilize, so offering a set amount makes far more sense in that environment.
This is the kind of stuff real hosts who don't oversell have to think about every time they provision and populate a new server.