Living in the moment means that you are fearless. You don't depend on the experience of the past ( you are innocent) and the future doesn't bother you either.
You knew that you had nothing to eat, and ground to sleep on etc, but inspite of all that, when all that becomes customary, and you are not depressed about it anymore, you are not scared, you will find joy in some way or another. Inspite the fact that you are so destitude.
Thus you have no alternative but to live in the moment, because only the moment will provide joy. The past was horrible, and there is no certainty in the future, future is no source of comfort. Only the present remains.
Be it with drugs, or just someone handing out some food to you, you will be immensely joyful. Even more joyfull than the people living in big houses. Sometimes you even pity them.