Are these people waiting for the rebels ? Doesn't look like this to me.
How many people are there on this picture? 1000? 2000? Mariupol is a city of 500.000 citizens. Ask yourself, how many of them might be actually waiting for the rebels?
If there where so many why didn't they come out?
Why was the city so peaceful before and now hundreds are fleeing the city?
The Russian backed rebels might go and conquer all Ukraine do you really think all the people there support new russia?
A lot of them just hate their current government and oppose it like a lot of Americans do. But would they change it to another one that might prove even worse?
Basically you leave them no choice. It's just a forced replacement in which they won't have a word.