It is my understanding that in the US 100% of EMV transactions (contact and contactless) are expected to be performed online. If so, why would a payment gateway be requiring a US terminal to support offline PIN (if online is offered) for CVM?
The short answer is international interoperability.
Not all countries joined the EMV band-wagon at the same time, and hence different countries support different types CVMs. Many markets still only support Offline PIN, or even Signature.
Hence, both cards and terminals support multiple CVMs, but do so in a prioritized order. The idea is that the card and terminal negotiate the highest common CVM that both support, in order to carry out a transaction.
If the American terminals only supported Online PIN, then travellers might fail to transact there, since they their cards might not support Online PIN. This could be a valid decision, but in some areas (like retail at airports) this is unacceptable.
Similarly if Americans went to overseas to countries where Online PIN is not supported, they'd expect their cards to support either Offline PIN or signature in order to facilitate payments there.
This gives most customers the best chance of at least transacting -- albeit with a 'lower' CVM.
From EMVCo, LLC
Chip Technology , A Guide to EMV Version 2.0 Copyright © 2014 EMVCo, LLC. All rights reserved.
“The second feature is a new CVM of offline PIN. While the use of offline PIN is entirely optional, it is possible to use an EMV chip card to verify a PIN entered into the terminal PIN pad by the cardholder and therefore provide a PIN based cardholder verification method available for all offline and online transaction acceptance environments, including below the floor limit and offline only.”