I presume that you are referring to things like threatening to deny the Bajoran pettition for membership because of their practice of a caste system? Or their unwillingness to allow Roga Danar's people to join until they had settled the issue of the mistreatment of their war veterans? Or perhaps the Timicin incident?
There is a common social and moral standard that a society must agree to in order to become a Federation member. Each society is allowed to decide for itself if it can accept those social and moral strictures. The Federation, however, has NEVER forced anyone to join it.
Of course, that begs the entire question of whether or not all value systems are of equal worth to begin with. Is it acceptable to say "It's their way" when a society practices racial or caste discrimination? Is it right for them to look aside when a society kills otherwise healthy and productive citizens just because they reach a certain age? Is it right to say nothing when a planet essentially makes slaves out of a certain portion of it's population?
A few examples of what I was thinking of....
TOS - The Return of the Archon
s - They pass judgement on the concept of the Festival because it's not what Humans would do.
TNG - Suddenly Human
- They simply assume that Jono would natuarally want to return to his Human family. They also look down on the Talarian concept of taking war orphans and raising them as their own children.
DS9 - Just about any Starfleet officer's, other than Sisko in the later seasons, reaction to the Bajoran religion.
VOY - False Profits
- Janeway, using Human standards, flat out rejects any concept that the Takarian religion might be correct or in any benefical.