^
Which makes the "Federation expanding in all directions" as that Romulan in TNG season 1 described it all the more puzzling.
You seriously do need to quit listening to Romulan propaganda
Typically propaganda is aimed at the masses. Not your opposing counterparts who know enough to contradict such statements if they're not true.
You're assuming the romulan doesn't believe himself the propaganda. Considering he's military, he was indoctrinated for years during training with just such propaganda.
Actually, combined with internal repression/punishments (tal shiar), propaganda is quite effective at keeping the population in line.
Just look at the japanese/german populations during 1943-45.
In Germany's case, there was some resistance - after its members agonised for years about the morals of killing Hitler, the attempts fail (dramatised in a movie with Tom Cruise). Hitler kills a few thousand officers, suspected of being sympathisers.
The result: no one even looked funny at Hitler after that - even when being sent to death for nothing, the war being clearly lost and many elites being in the process of making their escape.
Once a world develops the warp drive, their culture is bound to be corrupted by somebody. May as well be the Federation.
Culture being corrupted is non-sense.
There's no such thing as a pure, clean culture to be corrupted, dirtied, lessened.
Cultures are changing, mutating entities in their essence.
But the 24th century prime directive assumes cultures are immaculate and must not change, that interfering will cause mass suicides or otherwise dirty the cultures. Sociological non-sense.
Another poster correctly remarked how concepts of hygiene are erroneously being used by 24th century trek in conjunction with cultural contact.
If that's the federation's credo, it, by definition, should not go out exploring and take the chance of "corrupting" other cultures -
meaning, yes, the UFP expansion is at odds with its own ideals. It should be isolationist to a ridiculous degree, afraid to even respond to hails for fear of contaminating the caller.