Which is good in part, you look at Mac OS, it's an controlled environment. The standards are all defined within Adobe/Macromedia. No one can come in and change the way actionscript is being interpreted without Adobe.It sounds like you may misunderstand what the word proprietary means based on your comment. Proprietary: "protected by trademark or patent or copyright; made or produced or distributed by one having exclusive rights."
The swf file format is not protected. You can actually come up with your own editor to produce swf file, but however, it is prone to Adobe changes and it's not guaranteed.