In Europe, U2 were only playing bigger cities during the last tour. If you asked me, not enough concerts. They could have easily sold out more. In the US, they are playing much more venues. If they found a balance between "playing a couple more shows in Europe" and "playing less shows in the US" I'm sure they will still sell out all of their shows.
If promoters want the shows to sell out, they'll sell out. But of course, that depends on a lot of factors. Attendance can be adjusted based on what the ticket prices are set at, in relation to what the gross is. Example:
~ If market A's gross is $1 million and the ticket price is $100. Then 10,000 tickets will be sold
~ If market A's gross is $1 million and the ticket price is $50. Then 20,000 tickets will be sold
~ If market A's gross is $1 million and the ticket price is $25. Then 40,000 tickets will be sold
~ If market A's gross is $1 million and the ticket price is $12.50. Then 80,000 tickets will be sold
Etc, etc, etc,.
That's how the public react to ticket prices. That's how it works.
The average per show gross on the North American legs of the
Vertigo tour was around $1.7 million. And when they hit the road next year, their average per show gross on the North American legs will be about $2.1 million. This is because a) they will have sold roughly 10% more albums than the last time they were on the road here in 2005 and b) inflation will have increased a little over 10%. So, at the prices they're charging, there's no way U2 will do a full stadium tour in North America anytime soon, as they would obviously need to at least double their per show average gross here to sell out or fill stadiums from coast to coast...
And yes, "technically" you can still get tickets for a sold out show, even I got tickets for sold-out shows, but I go to a lot of concerts and I have some experience in that field. With some acts, tickets are just more easily available than with others, and U2 tickets certainly aren't easy to get. Simply because they sell so many in such a short time.
In some cases, yes. But U2 are no exception. On the opening night of The Police's
30th Anniversary tour in Vancouver, it was
extremely easy to get tix for well below face value. That and their second show grossed more than U2's two
Vertigo tour shows did in Vancouver.