I found this info on Apple's info page for merchants (About Apple Pay for merchants - Apple Support):
Apple Pay Cash transactions are processed like standard Apple Pay transactions.5 If you already accept Discover debit contactless transactions today, you should be able to accept Apple Pay Cash without any changes. If you’re not already accepting Discover debit contactless payments, contact your payment provider to make sure that your point-of-sale setup can accept Apple Pay, is configured with the most recent kernel and firmware versions, and can support the proper BIN ranges.
So, companies have to take steps in order to accept Apple Pay Cash even if they already accept Apple Pay, if they don't currently accept Discover debit contactless payments. If a business has the Apple Pay logo at the register, the customer has no way of knowing if Apple Pay Cash is accepted. That's frustrating. I hope businesses quickly take the steps to ensure they accept Apple Pay Cash.