I’m not clear where ethics fits in here. Was that expressly limited? They’re offering you an affiliate link to drive conversions - they’ve externalized sales to you - what (more) permission do you need from them?
They’re paying you to get real people to sign up with the intention of using the service. If you sign up a bunch of sock puppet accounts, or pay a bunch of people to sign up for a month and then cancel, then you aren’t holding up your part of the (perhaps unwritten) deal
That would indeed be unethical. But not passing along a referral code and subdividing profits from there.