...why not drop a few hundred bucks on a single PPC box that you can start a GCC make on, and let it run for a week if need be?

Because it doesn't work that way in a business. Perhaps if you're coding something with friends at home, you might be able to compile for a different platform (although I have reservations about any code "just compiling" on another platform), but in a business, the story is different. Businesses operate according to business plans and those plans include goals such as "produce software that exceeds market expectations for quality, speed, and documentation" and so on and so forth. If a business decides to release for a platform, they normally have to get tech support staff trained to support it, write documentation, update marketing materials, etc. These things all cost money and they must look at the ROI to see if the benefits exceed the cost. Maybe in your opinion they do in this case, but it is normally more complex than "simply compiling" for a different platform.

In addition, most development organizations of any significant size have standards and if GCC isn't the standard (and I doubt it is for Macromedia) then they must see if their current tools support the required action in order to extract the highest ROI from the original tool purchase. If those tools do not meet those needs, then they must get some staff members to evaluate new options which normally includes examining open source software in addition to visiting with sales people from proprietary vendors. If they decide they like the OSS tool, then the legal department normally needs to approve the license for that particular software to make sure that it doesn't negatively impact the business goals associated with that product. And the list goes on...