They only charge the tax if you use Play's IAP feature. On Android there is no requirement that you do so (see the Kindle App debacle -- it includes a store on Android but not on iOS). Apple does not allow purchases within apps that don't use the IAP feature.
If you sign up for this outside of the YouTube app on iOS you'll be charged the normal rate (and it will still work just fine on iOS).
Developers offering products within another category of app downloaded from Google Play must use Google Play In-app Billing as the method of payment, except:
* where payment is solely for physical products; or
* where payment is for digital content that may be consumed outside of the app itself (e.g., buying songs that can be played on other music players).
Hmmm, except I can't play YouTube videos outside of YouTube and Play Music doesn't give me access to the raw MP3 files, therefore the second point wouldn't apply. I think the difference is that Google isn't proactively enforcing this rule so for example, the Kindle app can link to the Kindle store even though that's technically not allowed.
You can use the youtube app on your phone, or the browser on your laptop, or the youtube app on your tv. So it isn't content that can only be consumed in the app on your phone.
If you sign up for this outside of the YouTube app on iOS you'll be charged the normal rate (and it will still work just fine on iOS).