On the forums, CM Rick Heaton explained the change: "The epic story in Helm's Deep is very tightly entwined with the epic battle system. The story in Helm's Deep is the epic battle we all know from the books. We've returned to the core story line from the lore and that line leads in a direct path, laid out by the Professor himself, into Helm's Deep and the battles and heroic deeds that occur there. As a result, this means the epic story requires the purchase of the Helm's Deep expansion."
"Epic Story Guy" MadeOfLions also chimed in with a much more informative post about the decision. "It's a change that serves to reward the players that do pick up the expansion, and makes the expansion a better deal, more worth your time and your money," he writes. "This is just an expansion change for now -- I'm hoping we can keep the epic story free in other updates."
Speaking of Helm's Deep, Turbine just posted the Champion class update dev diary for all to peruse!