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Tip fees were primarily meant to make sure someone didn't send themselves a large tip from one account to the next.

However we have a more important goal of having people think of PeakD and Hive as a place where they interact directly with their connections. The reward pool has tons of dependencies on the rest of the hive community... we need something that is one to one... direct action between one user and another

What's the problem with large tips? Do they promote the post?

Because otherwise it's the same as a transfer.