If the airport were to "close to the public" for any reason he could be considered as a Trespasser.

Heathrow (for example)[0] has a problem with homeless people sleeping there, if they're warned by security staff and found again then they're considered trespassers, and trespass is illegal.

[0]: https://www.youtube.com/watch?v=kdAVn5JWkhs

When poverty or existence becomes a crime.

It's not, and that's why the Indian authorities are letting him stay there.