An example is why Warren Buffett got special permission from SEC to withhold reporting his trades for longer periods then normal. If you know that WEB bought a stock today, then you know he’s done the research and it’s a clearly profitable trade to buy and hold it for the long run. If WEB had to report trades daily, he’d be out of business, he’d never be able to buy more than a tiny amount of a stock in a single day, because the next day its price would have increased at least 20%.
Selling any kind of successful trading system is to invite others into your trades and make them less profitable for you. Selling the system only makes sense if the system is unprofitable or it's so thinly profitable that selling it to suckers is more profitable.
Also, the implied premise here seems to be that nobody in their right mind would give up on something that made them money, hence it has to be a scam. I'd say that open source in general is a perfect example of people giving their work away for free for whatever reason. Maybe they would rather have fame (i.e. github stars), or maybe they want a developer career and see this as a way to get feedback on code they wrote. Or maybe they simply feel that the potential value they could gain from this wasn't high enough to make it interesting so they decided to set it free.