Earlier quoted context omitted.
There are several reasons that "signing" is the next, big logical step. Larger venues (most of which are owned by Clearchannel) will not give you the time of the day if you don't have a label behind you. It doesn't matter if you consistently sell out of venues a tier below them. Their booking agent won't even take your call. Sell a million mp3s of your song online? If one of the subsidiaries of the Big Four aren't be…
I've worked with multiple acts (Clark, Broadcast, Plaid) that have played (and sold out) the Bowery Ballroom without major label support. There is a very successful tier of well run larger indies like Warp, Domino, Beggars Banquet. They've worked out that not fucking your golden geese is a good long term play. But yes, even they struggle with the costs of big tours. Touring is expensive.
edit: and most importantly don't forget XL Recordings, who have Radiohead, the Prodigy and the White Stripes among others.