There's no catch to having a blog and receiving tokens from upvotes.
When people cash out, they're receiving money that other people used to buy STEEM. That's the catch - it's a transfer of wealth from people who bought crypto to people who either didn't buy it or who are making capital gains from it. This is why people who paid for STEEM are so eager to have more people buy into the crypto - to drive up the price so that they can get capital gains. There will be losers - that's guaranteed!
This is a good answer ... I'll make sure I'll remember this the next time I come across individuals like these.