This all boils down to poor practices and false assumptions by Flash developers.
Just because it's compiled bytecode doesn't mean you can hide secrets in it, that's security through obscurity.
And always use POSTs for actions that modify data on the server, never use GETs for anything except requests which only retrieve data (and don't modify anything)
It's even simpler than that -- don't let unauthenticated users break things! Google is not stealing someone's password to get to your site, they are just clicking links. If that can break things, you need to fix your application!
(BTW, GET requests can have side effects in practice; a GET updates the logfile, potentially filling your disk, for every request.)
In general, we don't hide secrets in Flash because it's easy to decompile. We already knew that.
To help real developers understand why this is a problem for Flash developers: Imagine the problems you'd have if Google suddenly started to combined your Google Code Search results with your content search results.
Just because it's compiled bytecode doesn't mean you can hide secrets in it, that's security through obscurity.
And always use POSTs for actions that modify data on the server, never use GETs for anything except requests which only retrieve data (and don't modify anything)