Nice question. Ok so first of all, places like starbucks usually have an open network with no password required to connect. After you connect to the Wi-Fi, they ask you to login via their site.
Now, assuming it was password protected and you uploaded the password in the app, yes everyone who wants to connect to that Wi-Fi and use the app will have to pay you. Someone who is not having the app can always try to get the password by asking the staff. Hope that clarifies.
So there's some kinda loophole here.
Using starbucks as example here - starbucks won't know that you are earning some cryptocurrency from their wifi services if you are the one who uploaded the password. But it will affect their wifi performance owed to others that supposedly are not connected to them.
I bet that shop would not permit their wifi connection to their non-customers. So the scenario would be the same to other shops/stalls that allow their wifi services to their customers only.
It's perfectly fine for malls or airports as they are really catering the public.
Yes this is possible, but businesses that are very particular about this, usually keep changing their passwords consistently.