'Congestion' charging

I know I’ve asked this before, but what is the point of congestion charging? I see that there are new plans afoot for a wider charge to be imposed:

“Transport for London realises that charging on congested routes will encourage drivers to find toll–free rat runs. It plans to tackle this by introducing more traffic calming, including road humps and chicanes”.

Why is this appropriate if the aim is to reduce congestion? A charge is imposed on certain busy roads, so people unsurprisingly look for alternative routes where there will be no charge, thereby achieving the allegedly desired aim of reducing congestion on the busy roads with the charge. But TfL then treat such people as if they’re committing some kind of tax evasion by doing so. They employ pejorative language to describe them — “rat–runners” — and institute measures deliberately to force them back onto the roads that charge, so they’ll have to pay.

I know I’m just shooting fish in a barrel here and the aim is obviously not just to reduce congestion but also to raise revenue to fund the administration costs of congestion charge itself and to pay for other projects in future. But the hypocrisy of the way it has been presented to London voters really winds me up.