My understanding is that Revolut is looking to become a full-fledged online bank in the UK (and probably the US too). It brings a different set of expectations with regards to what's offered than, say, simply being a travel-focused prepaid card. Considering that PIN is actually a fair bit of hassle in the US (because many smaller businesses and restaurants never bothered setting themselves up for the possibility that a card would need one), Revolut may not want the complaints from their American clients.

Also, to satisfy Dodd-Frank's debit routing requirements, Revolut will likely have to set up their cards similarly to what other major US banks do*. As a result, even if PIN were to be asked for at a store in the US, it's very possible that the store will let you bypass entering it (if they don't flat out do it for you).

Of course, they may still decide to set it up as chip and PIN (for foreign transactions at least) despite all of the above. We'll see.

* Right before submitting this post I realized that Revolut could be exempt from this requirement since they might not have enough assets. However, they'd be silly IMO to not set themselves up for the possibility of breaching that cap.