There are a few prevailing theories as to how it happens and how people get around it.
The most popular, although not entirely likely considering they are competitors, is that Ticketmaster illegally sells the tickets directly to stubhub and gets a chunk of the resale profits. It should be noted ticketmaster has their own resale methods within their site and some people
blame them for taking all the tickets and only releasing a small portion to the public.
This still wouldn't explain how they get around the credit card requirements, other than maybe some tickets are listed without that requirement.
The other is that scalpers use bots to buy all the tickets. Again, no one seems 100% sure how this gets around the requirement of ID and card at the door.
The main thing is that it is not very practical to check the tickets and IDs of 5000-10000 people at the door of a concert. In all likelihood 90% of people don't get their tickets checked. In my experience this has only ever worked when the tickets were actually picked up at the door as you enter through a special fan club or presale entrance. That's always an extremely small part of the total crowd though.
I've heard that the whole ticket and ID thing is fake anyway. The only time it ever works is if you are using will call. I've never heard of a band/promoter/venue actually making every single ticket holder show ID and CC at the door. It also isn't practical, sheer numbers and delays aside, because of the amount of people that go in groups. How would you get in if you were meeting the holder there and he got in already?
Also to remember, and no disrespect to the Hip, but most bands love scalpers. While they often publicly decry their actions, secretly it is great for the band. It increases demand and allows them to almost guarantee sell outs by creating a market for their tickets. What i mean is, some shows won't sell out but the perception is that you, as a consumer, will be battling scalpers and that you must buy tickets right away. Then the artist gets to act all high an mighty and say they had nothing to do with it, when for all we know it is actually them and their promoter selling the tickets to stubhub for a profit. Meanwhile they get to cry foul and say that they feel bad that their fans are getting screwed by the system. But don't worry, they'll add a second show! just for you the fan. Not all bands are like this but a lot are.