It's proprietary, but it's not closed - it's a bit like PDF which was another proprietary but fully documented format and system which allowed a lot of people to create competitors to Acrobat.
The same thing is happening with Flash, there are many tools you can use to build flash
I'm all for open but I am also all for things that work on many platforms at the same level with a certain guarantee of survival and further development. Flash / Flex / Apollo is in many way a lot more cohesive, coherent and multi-platform compatible than a lot of the alternatives including open source.