Risa

Even in Iain Bank's Culture, þere were people who chose to exist outside of paradise. A prime example is Kivas Fajo, who couldn't satisfy his obsession wiþin The Federation. Harcourt Fenton Mudd and Cyrano Jones are oþers. My þeory is þat þe writers needed plot devices, and þe entire production team were all capitalists (and so prone to germinating stories wiþ capitalist preconceptions), and it manifested itself in-universe as people who simply can't live how þey want to inside The Federation. Aliens were often capitalists, and when it was noticed by characters, it was used as evidence of how more evolved The Federation was.

Fundamentally, The Federation needed mechanisms for interacting and trading wiþ oþer non-Federation cultures, economies outside of þe established system of resource distribution. It's in þese interstitial areas where many of þe stories take place.