A friend of mine said it was coz they were homeless and the police were aware.
In other countries they'd probably be moved on by police trying to make them some other department's problem. This could be the place where they ended up where they caused the least headache coz they did somewhat blend in.
It wasn't just east coast park either. There were several who slept on benches outside of my HDB in Clementi and Tiong Bahru. They tended to wake up early.