Q: What happens if you let everyone contribute to and take from a Starbucks card? A: We are in the process of finding out, and the answer wasn't what we expected!
Why isn't this just an interesting, unexpected result? Furthermore, condemning this guy is like condemning Lulzsec for exposing security vulnerabilities. Do you think that no one would have figured this out if OP hadn't? He's exposing the vulnerability and sending the proceeds to charity, would you prefer that Mr. Unscrupulous just silently steals it all?
That doesn't make the misuse of it okay.
Just because it's easy doesn't mean it's okay. I can reach in and grab $50 from the tip jar at a restaurant while the cashier's back is turned. I can walk into most stores, stuff something under my jacket, and walk out.
Just because it's easy doesn't make stealing okay.
Both of your examples of ignore that there is an "appropriate" level security for those contexts. The tip jar is in the plain view of other diners and staff; the store has staff and video surveillance. I'll leave it to others to decide whether an unsecured cash account accessible to millions of anonymous users has appropriate security in place.
I think a more accurate scenario is: Q: What happens if you let everyone contribute to and buy coffees from a Starbucks card? A: We are in the process of finding out but some very smart but unethical folks are pulling funds outside of coffee purchases and ruining the pot for everyone else.