Don't scare the Horses!

[Cobaltblue]

No it's not, it's about money. If it was about air quality, they wouldn't say the same cars can come in but pay for it - you'd not be allowed to drive in. 

That's not really the case is it.
As you have to pay a fine for go within ULEZ with a non compliant car it reduces the number of such journeys.
It would be unfair to totally ban someone with a Non compliant car from traveling within ULEZ once a month say.
Its the daily users that the fines are there to dissuade. 
If there were no fine then there would be no incentive to get a compliant car.
My eldest son had to replace his car to be ULEZ compliant (Lived in Mitcham) he sold his car privately to someone who didn't need a ULEZ compliant car and bought the same make and Model but a little newer.
The difference in value of the two cars was around 6 Months worth of ULEZ fees so it worked out for him.

Cheers

Mike T