There can be exception which is in sentence where there is nothing following after preposition. It is sentence "Já jsem pro" and it basicly means "I am for it/I agree with this suggestion" While actually you feel there was originaly some word in accusative after pro, it vanished and "pro" word become something else than preposition.
Oh! Actually in spanish ther's something similar. We don't use "pro" at all but... When we want to say "Upsides and downsides" we say "Pros y contras" (pros is plural of pro). And it's the same. Contra means "versus" in spanish, an here it become a noun... Maybe did we use "pro". Can it be a latin trace? Even in czech?
For Polish speakers: "potrzebowac" needs genetive in Polish, right? So in Czech it's accusative instead?