We covered the AppleBot directives documentation before, back in May. But what I didn’t know is that AppleBot is unique in that if you do not make any mention of directives for AppleBot but you do for GoogleBot, it will follow the GoogleBot directives you set up. Bing, Yahoo, etc, do not do the same, they follow general directives or directives detailed for their bots but not for GoogleBot.
John Mueller said this on Google+:
If you have a well-crafted complex robots.txt file (e.g., longer than two lines), keep in mind that Applebot will follow Googlebot directives unless you explicitly set up a user-agent section for it. I’m not aware of other crawlers doing something similar, so I thought it would be worth sharing.
I didn’t know it was a unique thing but John would know and he mentioned it.
The official AppleBot page says so:
If robots instructions don’t mention Applebot but do mention Googlebot, the Apple robot will follow Googlebot instructions.
This is specifically for Universal Search and App Deep Linking I believe – at least for now.
Forum discussion at Google+.