Good Question ... the ONLY way to SELL software, is through iTunes. Apple is going to get their cut (30%, I believe). This is for the mass distribution software and books, etc. Stuff you want the general public to consume.
However, say a company like Delta Airlines wants to put their aircraft checkoff lists on the iPad (which they have); they can create an in-house solution to install on Delta-only equipment. I don't think Delta would want the general public having copies of all of their aircraft manuals and check-lists.
Even if they can do it internally, having to get Apple's approval of the app first is both a security risk and a potential development bottleneck. Our internal developers had to invent new epithets after experiencing the approval process at Apple.
I often see this figure quoted as if it was robbery. I imagine it would cost a lot more than 30% to make any product available to people around the world, by any other means. When some moron can make a million with a fart app what would they complain.