I think that spot-fixing is probably absolutely rampant across cricket, especially in limited overs games. It might not be as prevalent at international level but there's still probably a lot of it. Take a look at this list: https://en.wikipedia.org/wiki/List_of_cricketers_b...
And then add in Mark Waugh and Shane Warne, who got slaps on the wrists for passing on info to bookmakers (probably got off lightly because they were massive stars - other guys have been banned for the same thing), and the weird circumstances surrounding Bob Woolmer's death, and it starts to look like what we do see might just be the tip of the iceberg.
Cricket is a game which is really well suited to spot-fixing, and spot-fixing is a lot harder to identify than match-fixing. Spot-fixing is also potentially easier to justify to yourself as a player, because you're not throwing the game as a whole necessarily.The big markets for cricket viewing are riddled with fraud and corruption generally, and gambling is used to launder funds by criminal gangs there. Most cricketers don't earn huge amounts of money so the added incentive of a bit of cash from fixing could be appealing to some.
Basically there's a perfect storm of bookies wanting to fix, easy ways to fix, and players willing to fix. It would be naive to think that the identified cases are the totality of the problem.
Cairns may or may not have done it (I think he did but I can see why a jury acquitted him) but I reckon that there's a lot more to this issue than just him and his chums...