Indeed, a caste system is obviously far worse than arranged marriage. But if the internal Federation principles are already weaker than human rights we can expect that the standards for dealing with non-Feds are even weaker.
To stay with the Bajorans, the Feds do neither apply any pressure on them nor do they condition their help upon an abolition of the caste system. And it is not like we are talking about peanuts, a caste system means after all that entire society is built upon exclusion. It is antithetical to everything we believe in.
Of course the Feds create incentives, change yourself and you can join our club, something which is similar to the behaviour of the EU, but this is not the same as setting standards for outsiders. And this slow expansion and soft convincing is the best way to give your values material reality in space. Lecturing other species on how to behave or not communicating with them because you do not like their ways will not make them change.