Thanks for the response Matias. This is an important discussion to have!
Of course, YouTube uses a certain share icon and for the sake of consistency, its users shouldn't be confused as to the function of a button when they pick up a YouTube app on any platform — I get that.
But, consider an iOS user new to YouTube (or any of Google's other properties). Perhaps they don't use YouTube on the desktop, only iOS. What will they think that share button does? Do we want them to find out by trial–and–error, or would we hope that it were clear on first glance? I'd say we'd both agree that the latter is preferable to the former.
And that scenario might seem farfetched but in reality it isn't; I know people whose main computing device is an iPad, and whose only computing experience is with iOS. So while the share glyph which Google is using is
commonly used in Open Source projects and other web properties, on iOS, it is uncommon.
The majority of iOS apps follow Apple's HIG and use Apple's share icon. There's a reason Apple bakes it into iOS — they want you to use it.
So while we might disagree with Apple's design choice, we have to respect their decision, and more importantly, adhere to their wishes. Not for their sake, but for the benefit of their users. Of course, Apple doesn't do the same vis–à–vis iTunes on Windows, but that's a whole other discussion.
You were right to begin with; platform guidelines are important. I don't like seeing Android apps which look like iOS ports, and vice versa. But if you're going to admonish Yahoo for making some questionable design choices with Weather on Android, it's important to note that Google isn't a saint on iOS either.