Zelle is an app rather than a protocol, which is why e.g. PoS terminals and web checkouts don't support it.

Zelle, from my understanding, is a “protocol” – many banks support it in their own apps. (Scare quotes because I’m not entirely sure it’s a proper well-defined protocol.)

And there’s nothing preventing PoS from supporting individual wallet apps – see e.g. WeChat Pay / Alipay in China. (Alipay+ is also a “protocol”, i.e. other banks and wallets support it.)