`this` being dynamically bound is a feature, it's broken as intended. You can always `bind` a function to remedy this if it's a problem for you.
Re:TypeScript 0.9 released
by lucian1900 on Wednesday June 19, 2013 @05:10AM
(#5904531)
Why would it be a feature? It is almost never the desired outcome, thus a terrible default.
Btw, TypeScript also aiming to fix context problem so in callbacks 'this' keyword is always related to parent class.