You do pay extra for it, by way of increased programming rates. The $6.00/mo quote is a bit high, but it's definitely in the $4.00 range with ESPN-HD. You've got ESPN at $2.25 to $2.40, ESPN-HD at ~$0.80, and the other ESPN channels at $0.20 to $0.30. This is all figured into your cable bill.
Content providers are not regulated like that; they are not monopolies. If Disney wants to sell ABC, ABC Family, ESPN, ESPN-HD, ESPN2, ESPN News, and ESPN Classic in one big package bundle, that is their right. They also have the right to dictate the terms of carriage in their contract. In Disney's case, they require a clause in all their contracts that says ESPN, ESPN2, etc will be in the basic programming tier. They won't sell these channels any other way.
How can VOOM offset the extra $4.00+/mo for the ESPN channels, without significantly increasing rates (or increasing rates at all)? One option is to eliminate the choice of a free premium package (HBO, Showtime, etc) in the $39.90/mo plan. Ooops, they already did that.