They are not, but many DO have mental illnesses. Out of their control, but it doesn't change the fact that any 'help' you give is likely to be squandered.
My girlfriend's sister is a psychiatrist that deals with homeless people regularly. She said about 8/10 homeless people she encounters has some sort of moderate to severe mental illness that prevents them from making responsible decisions with regularity or holding a job.
oh absolutely... i'm not suggesting that giving them money is necessarily the right thing to do... but to sit on a high-horse and say everyone who gives people money is an idiot and that somehow one is helping them MORE if you don't help is misanthropic.