For me it was a clear message to the outside world, how little backing Dart has inside Google, if their management allows for choosing a programming language designed by the competition instead of their own one, for their own products.
Or it shows that they see that there's already an established base of people using TypeScript, more than Dart, and choosing to follow where the community exists. Throughout the development of Angular 2 the team also talked with the developers of other frameworks, libraries and tools, which shows that they believe in the community more than trying to push their own agenda.
You saw "allowed" like it's a bad or unusual thing. Allowing is the way Google works.