Ah okay, I understand now why it didn't make sense to some people - I do see the potential problem in the scenario you describe. So, I'd be open to a cap. In my mind, I only know people who work minimum wage jobs (essentially) so when they get a second job, it feels disgusting that they have to continue paying tax when they're working on their days off.
Personally, I don't need there to be a cap. As far as I'm concerned, those two poxy days we get out of seven to ourselves.. whatever we choose to do with that time should be separate from the government and not required to be disclosed.
Days off mind, not side jobs during the week. And who is earning 20k from a Sat/Sun job??
Can we not focus on one or two extreme examples, and just focus on what the majority experience and think about them and their real-life needs? People doing bar work, for example, at the weekend because they need extra money - their full-time 9-5 doesn't pay enough to cover bills. Why should anything they do on their days off have anything to do with the government.
Maybe we just need designated / declared work days and days off. There is a way to build a fairer system, just needs people brainstorming that rather than excuses for why not to do it.