Yes, bounty is an investment of time and effort.  But I see so many people signing onto bounty programs without analyzing the project.  As with any investment, you need to analyze whether or not a project is worth your investment in time and effort.  I've previously joined airdop and was receiving bounties but as I found more about the project, I was convinced it was not worth my investment.  I left that bounty program quickly.

I also found out that participating in the bounty program forced me to learn more about cryptocurrencies.  So the received reward is both knowledge and bounties.