Re: Whatif 17.5 million people sign the petition?
"The UK voted."
At least some of them appear to have put the pencil in motion without engaging the brain. Now they're discovering that they were actually doing something real - maybe they thought it was the same as a Facebook "like".
The basis of the petition is that a referendum should require a reasonably substantial majority to change the status quo. There's also the point made by another commentator that non-resident UK citizens were denied the vote and yet they are particularly likely to be adversely affected.