If this is true, I feel that it was a terrible thing to do, but I'm also convinced that we haven't heard the whole story. It's important to understand that most homeless people are human beings and probably have families, but a lot of them have very serious problems. When you work in a retail environment and deal with the homeless each day, like I did, you see that the negative stereotypes that are allocated to them are usually true. They aren't hygenic, they talk to themselves, they're usually very angry and are only one wrong word or gesture away from showing violent behavior, and they're disruptive and either beg people for money, or ask you to give them something for free.
I'm sure there are some real dicks out there who accost and harass homeless people for no reason, but I think it's important to give Starbucks the benefit of the doubt here. It's a private retail environment, and they do have the right to refuse service to anyone. Due to lawsuit concerns, I'm sure that isn't a responsibility that most employees take lightly. My personal policy towards the homeless, if I owned a business, would be the same as it would be for any other customer, with one exception: If you emit a foul odor (like feces or urine), sorry, but you're outta here.