In a few books I've read that the Athenian/Delian League was a voluntary coalition of city-states.

However, when Naxos (and later Thasos) left the league, the league responded by attacking the city-state, tearing down their walls, removing their ability to have a standing Navy, and forcing them to continue paying taxes (without any voting rights in the league). Also, Athens who just so happened to be at the heart of the league, seems to have had a habit of using the treasury to finance Athens construction projects which calls into question how egalitarian the league truly was.

This leads me to my core question -- was the Delian League truly a voluntary coalition of city-states, or was it just an Athenian empire with clever marketing?