That was a bit surprising but maybe... There were several different species of pokemon there. Even species that normally compete in the wild. It sort of felt like... that hot spring was an area they had all designated as a peaceful place. No matter how much they compete in the wild, in that area, they all share. I guess, if it was something like that, they felt that things should also be shared with us. Even if we were just humans.

Though probably they wouldn't have been so friendly if we actually tried catching any of the pokemon there.