There are many reasons for
homelessness - not just substance abuse but also mental health issues, relationship breakdown, domestic violence, etc. My DD runs a local homeless charity which she co-founded and I asked her about street beggars. She said that it is a very difficult moral question which is hotly debated.
Her opinion is that there is no right or wrong answer. On one hand, she has seen many instances where begging is funding people's addiction and contributing to their personal decline. On the other hand, if they don't get money that way, they may turn to crime to get it. People won't start their recovery until they are ready, and until the resources are available to help them, and many will turn to crime if there's no other way to fund their addiction.
DD says that she knows quite a few people, usually ex-homeless, who are accommodated but pretend to be homeless. They always have a good sob story and it's usually to get money for drugs or alcohol.
Her advice is to only give to someone if you feel comfortable about it, treat stories with some scepticism and suggest other ways of helping, eg contacting the council's outreach team or a homeless charity.
I don't think that the big problem, at least in our city, is professional beggars. It's drugs, and to a lesser degree alcohol, which is contributing to the massive number of street beggars, whether homeless or not. Drugs are cheap, they offer temporary escape and the long term help for recovery isn't widely available. However, DD sees many people with serious mental health problems - some with addictions, some not - and the help available is virtually non-existent.
I remember looking after her kids over an Easter weekend whilst she was trying to help a woman whose home was a cupboard outside a pub where they kept the bins. This woman was an alcoholic with severe mental health problems, and was physically and sexually abused by her boyfriend (who had a flat and didn't live in the cupboard.) It took DD ages to coax her to come with her to hospital, where they kept her for a couple of days for her physical injuries but then wanted to put her back on the street because there were no beds available for mental health. DD practically had to beg until they found a place in the psychiatric care unit.
That's just one person and there are thousands like her. There's supposed to be care in the community but the resources just don't exist, so mentally ill people are liable to end up on the street. Many of them self-medicate with illegal drugs. So it's often not a straightforward case of substance abuse, there is serious mental illness too.
So, all in all, a very complex problem. Should you give them money? If you want to. Will they spend it on drugs or booze? Possibly. Is their sad story true? Possibly. Are they all addicts? No, some are not. Are they all mentally ill? No, some are not. Are Are they professional beggars? Probably not. Is there help available? Yes, but you have to know how to access it. That's what the charities can do.