Because Obama is regarded as a “rock star” President, and Blackberry and others want to publicly use the fact Obama uses their product.
Just like everyone knows if you wear sneakers with Michael Jordon’s name on it, you instantly become a better basketball player 
That’s why it’s an issue
I have to assume they are worried about tracking via cell-phone towers, especially if the Blackberry servers are in Canada. You might not need an actual location, just a verification he’s in, say, Chicago, and you could start guessing from there.
Looks like it’s not the USSS which is driving this issue; it’s Obama’s own transition team: http://www.cnn.com/2008/TECH/11/17/obama.blackberry.ap/index.html?iref=newssearch#cnnSTCText
Wasn’t there controversy in the past few years over some of Bush’s advisers (or perhaps Cheney) using Blackberries when they shouldn’t have?