You might think that the Prime Directive would prevent Star Fleet from coercing recalcitrant planets. But recall that ambitious captains like Kirk and Picard violate the Directive all the time, and almost never get punished for it. Recall also the Klingon spokesman who called the Federation a "homo sapiens-only club." And humans certainly seem to dominate Star Fleet - the face of the Federation for outlying planets.
They make up something like 80% of Star Fleet personnel an even an higher percentage of high-ranking officers (captains, admirals).
Worth noting, also, that secession from the Federation doesn't seem to be possible. Just look at what the Feds did to the Maquis, when the latter try to leave.
On a slightly more serious note, forcible assimilation into the Federation might actually be an improvement for many societies. In the one accession process we see in detail (Bajor), it's pretty obvious joining the Federation would stave off the threats of theocracy and reoccupation by the Cardassians (who are much worse than the Federation, even if you discount the latter's propaganda)..