Overselling with banks can affect everyone or it can affect a tiny minority.
Overselling with hosting can affect everyone or it can affect a tiny minority.
It depends how it's done entirely. The only difference as I see it is that the banking industry is highly regulated (in most western countries) and that in the web hosting industry people can practically "make up" numbers and put them next to a price with no intention of providing that service.
Overselling is what makes hosting and servers affordable to the masses. It's what makes broadband available to practically everyone, it's what makes mobile telephones available to everyone (do you think your phone provider has enough capacity for everyone to send a message all at the same time...? Think about New Years Eve).
Getting the balance right is the tough part - most mobile phone networks seem to have it together, because 99.9% of the time, you can call and send messages instantly with no "network busy" messages.