The documentary I saw said the Hijra are not permitted to work at all by the government, and that their religios sect prohibits prostitution. So they can only beg in the street. But the local religious and/or superstitious beliefs also provide that they have the power to curse others, so the "begging" basically entails threatening to curse the locals to extort the charitable giving.
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AKA Smapdi. IANYL. TINLA.