Also, it's probably somewhat of a "who cares anyway?" attitude. They're not gonna go pro, they're not getting tested, etc. Any kind of competition involves someone bending the rules, like it or not.
Age groupers are much more persistent, they have their hobby permanently dialed in, they wouldn't even know where else to search for a sense of achievement. They are used to getting faster from a combination of better technology and better training, and to some of them doping might seem almost like a natural extension of carbon and training plans when those are maxed out. Fortunately, the deep end of both the training and the carbon is so far out that I am confident that it's still only a small minority who falls for the temptation (the very few testing that has happened on the amateur level concentrated on the top results, if doping works, that's where you will find the dopers)
(source: getting more carbon and more training as I get older)
carbon -> carbon fibre bike parts, which is way lighter and way more expensive but makes you go faster with less effort
Triathlon bike rules are looser, but most triathlon courses have less climbing so aerodynamics take precedence over minimizing weight.
The race organiser does not pay for the testing directly. British Cycling takes a small levy for every rider taking part in a race and I imagine this goes towards paying for the random testing.
The UK anti-doping website has a list of all current and historical bans for drug test rule violations covering a wide range of sports:
https://www.ukad.org.uk/anti-doping-rule-violations/current-...