I mean, do you say "No no, you can't complain about all those chemicals being dumped into that river... look at all the value people are deriving from thing the factory is producing!"
Cars and drivers, in my view, push MUCH more externalities onto everyone else (and let's not limit ourselves to thinking locally, all that CO2 we emit from driving disproportionately affects those in the third world and those yet unborn).
I don't think we should be using the argument of 'well, the externalities for cars are worse' when deciding how to deal with the externalities of scooters, though. By that argument, we only have to ever make sure our externalities are better than the very worst offender in order to be beyond reproach.