It's fine that they are using their free time for such activities that can be profitable and productive for them. But, not all bounties today are very much worth taking. We all know that many bounties are total scam and don't want to pay attention to the concern of their participants, they don't even have plans of paying them with their participation so their efforts at the end are likely to be wasted. The concern with that is they should start knowing what are the legit and not.
Its good for them, while they have a freetime they learn a lot more about in bounty, and they can earn also an extra income. But they need to choose well if the project is not a scam. This time its very important for young people that they an income, even if its in a small of amount only, and they can use also for savings on their future.