It's nice that some want to give credit, change methods or whatever for their own reasons. But that's down to their own personal values. Morality, ethics and so on are not carved in stone. Different societies/groups have different views and none are 'right' in a universal, ultimate-truth kind of way. The law is a fairly useful indicator as to the moral values of a specific society, though.
We can choose to criticise those who use the ideas of others to make money (some well-known internet magic vendors spring to mind), or may decide that it's fair business practice. The law says that it's fair. Our group might disagree, in which case the next step would be to lobby an MP, I suppose. I'm not volunteering for that, by the way
We might criticise someone who works out how a performer achieves an effect (and then goes on to copy that performance) on ethical grounds; criticise them for a lack of originality; or simply not care. The law cares not. The public might, though, rejecting Derren-a-likes when they pop up.
If someone reads Corinda, understands it and produces 13 leaflets covering the same kind of material, that's up to them. I imagine the results will be less good, but maybe there will be some added value. Bringing it up to date might be fun. However, reviewers will judge that and sales may be affected accordingly. That's the market making the decision, not magicians.
Creative people in any sphere have to stop worrying too much about imitators and just get on with innovating. Hopefully consumers agree and spend their money accordingly.
What's the worst that could happen if you accidentally duplicate one of Max's methods? A grumpy letter, criticism from your peers etc... which some people won't care about. You obviously do, which is great. But I guess you're main motivation is not to make vast amounts of money. Very rich people tend to be rather more ruthless.