I guess it depends on what the streamer wants to call the donation tab is how they want people to think of the extra money given.
Definition of donation
: the act or an instance of donating: such as
a : the making of a gift especially to a charity or public institution
b : a free contribution : gift
Definition of tip
: a gift or a sum of money tendered for a service performed or anticipated : gratuity
Definition of gratuity
: something given voluntarily or beyond obligation usually for some service; especially : tip added a gratuity for the server
So if I give a extra money with the expectation that it will be put to some good use that will benefit all of the viewers, that is a tip or gratuity, but if I give money with no expectations then that is a donation, but then again that might not be how the streamer thinks of it.
Have we beat this horse to death yet?