It is nowhere near that easy.

http://100parkingtickets.com/

It has happened multiple times.

NULL, NV, XXX, MISSING, NO PLATE

I wonder if the system is also vulnerable to SQL injection via vanity plates.

I think this is unlikely since plates are alphanumeric. Although I suppose if you faked an image of a plate it's possible you could cause problems for a plate scanner.

[1');DROP TABLE tickets; --]

(Damn. Way too many letters...)

There was someone who not too long ago fixed an SQL injecction to his front bumper and it managed to break the automated toll collection system.

Relevant XKCD:

https://xkcd.com/327/