amok: It is a little more complicaterd. Humble Store uses reginal prices, but Humble Bundles and Humble Widgets uses flat global prices (US$).
Redfern: Its because its totally pointless to combine regional price and RWYW in one deal :)
It is a choice. Indie Royal, for example, uses regional pricing in the PWYW model.
Redfern: As for Humble Widgets...well, i guess its matter of time...or up to game developer to decide. I know that main real difference between Store and widgets is that widget give game developer much more control over game, while store totally shift all management (and right to use discounts for promotion) to humble.
Rights to do discounts is always up to the publisher/developer, not to store fronts. Stores can suggest discounts and promotions, but the final say is always on the publisher/developer, this goes for all DD stores.
Widgets are different in that there is no store front, they are hosted on the devs. sites only. Which is also why they are able to give back such a large % on sales via widgets (90%?), compared with store fronts, but the onus then is on the devs to do the marketing also.