I rarely go to SB (nasty coffee) but my wife enjoys a couple of their drinks.
She said they have posted a Ten Point letter of rules and conduct. (Nothing explicitly stating a must-buy-something policy, and nothing explicitly prohibiting loitering, either)
But the list is intended to warn patrons to behave well, lest they break some rule.
I can go to one of three coffee shops within 1.5 miles, all Asian owned and operated. Ordinary coffee,, and assorted donuts and pastries.
Owner probably has bag or box of day old donuts, if homeless come around. She gives homeless dry donut, and says: "You go now."