Even in modern society bystanders are obliged under law to render aide if it will not endanger their own life. Flint under the alias MR "Brack" wound not have been allowed to purchase an entire planet and also have the right to begrude passing ships lifesaving assistance if it did not endanger his life.
Flint was being a jerk and kirk wasn't going to lecture him or quote laws (which probably existed) he simply stated Flint could kill them and and ship would end up taking the required medicine anyway.
If a cop was shot and bleeding to death in the street and his fellow officers saw a towel on your clothsline that would stop the bleeding and you got on your porch and told the other cops they couldn't set foot on your propety and you would kill them if they did. That person would be out of line and I think that a cop ignoring that threat and being shot dead would rightfully provoke a response and them eventually getting the towell.
Starfleet as Kirk said "are the only police around" and they deserve the same benefit.
Guess what, they'd be happy to pay for it and most decent people would be happy to help.