Why did Apple lift its ban on apps written in Flash?

Steve Jobs once called Flash the No. 1 reason his devices crash. What changed his mind?

With a terse, five-paragraph statement issued Thursday morning, Apple (AAPL) reversed a five-month-old policy that had sparked an industry-wide debate, a government probe and tens of thousands of words of heated commentary -- including Steve Jobs' own April 2010 "Thoughts on Flash."

The newly inoperative policy had prohibited software developers from using cross-platform tools when writing apps for the iPhone, iPad and iPod touch. Covered by the ban were several popular third-party toolkits that make programmers' lives easier by allowing them to write one set of code that runs on a variety of devices. None was more important -- or controversial -- than Adobe (ADBE) Flash.

Apple's statement doesn't mention Flash by name -- and we are waiting for clarification from Apple and Adobe -- but as long as apps (or ads) written in Flash are compiled ahead of time and don't require downloading any Adobe code, they should pass muster. [Adobe confirmed this in a statement issued Thursday evening. See Adobe on Apple: Our glass is half full.]

Adobe's stock opened about 9% higher on the news.

What caused Apple to change its mind? The leading theories:

• Feedback. The company line is that Apple has "listened to [its] developers and taken much of their feedback to heart." Nobody is buying it.
• Competition. First from Flash-friendly cellphones powered by Google's (GOOG) Android, and now from the wave of Android tablet computers about to hit the market.
• Regulation. The FTC is known to be looking into Apple's ban on cross-party platforms, reportedly at Adobe's request.

Apple also announced that it is publishing -- for the first time -- a set of App Store review guidelines that should shed some light on what has seemed, until now, an arbitrary and capricious approval process. It's pretty blunt. "We don't need any more fart apps," it warns developers. "If your app is rejected, we have a Review Board that you can appeal to. If you run to the press and trash us, it never helps."

UPDATE: Without further clarification from Apple, it's not certain what the immediate effect on Adobe might be. Flash elements embedded in Web pages still don't open in the iPhone's version of Safari, although in theory that could change. "Here's one interpretation," writes Fast Company's Kit Easton. "What happens if you build a new browser app for the iPhone that has all the back-end code needed to run Flash in web pages already downloaded and packaged inside it? Result: De-facto 'full Web browsing' experience, complete with Flash, on an iPhone."

Forrester Research's Jeffrey Hammond is less optimistic. "It looks to me like [Apple's new policy] doesn't change anything immediately, as the way Adobe was targeting iPhone was to use the AIR compiler to cross compile Actionscript to directly target ARM APIs through private, non-sanctioned APIs. These changes still don't allow them to do that as far as I can tell, and they don't allow Flash to run in iOS."

"But what is does signal," he adds, "is that Apple feels it needs to be more transparent to developers and flexible in the options it gives them to keep iOS as the preferred mobile development platform -- the one that developers choose first."

See also:

• Adobe on Apple: Our glass is half-full
• Steve Jobs' Flash manifesto
• Has Steve Jobs gone mad?
• Why Is Steve Jobs Flash-obsessed?
• Apple and Adobe: Who needs whom?

[Follow Philip Elmer-DeWitt on Twitter @philiped]