[pupeno]

Apple requires the ERN because Apple is very US centric. For Google, there might be cases in which the ERN is not required because the app never leaves the US (because for other countries it comes from other countries).

Apple is required the ERN to cover their asses, I believe. The ERN is required by the US government, so, if you don't have it, you are breaking the law whether you are using Google Play or Apple. So, you should get it for Google Play too.

[pupeno]

To add to that, maybe Apple is more liable because of their promises to check for quality and problems in the store, while Google allows more things in it. That just means a change for them, you, as the app maker, are always liable.