Edited the whole question in order make my point be understood.
I know that check-in staff and destination immigration officials check the papers permitting to reach the destination country, generally speaking, but in a country I'm a citizen of, also exit immigration checks this thing for its citizens, and in general exit immigration checks only whether a person has stayed in the country legally, not necessarily the eligibility to enter the destination country.
The main issue doesn't raise up for a person having no papers to enter the destination country, because they'd eventually be blocked at check-in in the first place, furthermore few people are stupid enough to take a flight with no necessary papers because they know that they'll further meet destination authorities.
The issue may concern a person who doesn't want to show the other passport to the immigration officer not because dual citizenship is illegal (it is legal), but because they simply don't want to reveal any additional data of themselves to an istitution who doesn't need them or is not entitled to know them in the first place, because out of their jurisdiction.
Provided I have no pending charge and having another citizenship is not illegal, why should they hold me in case I don't want to show them the other passport or a paper granting me access there, if the latter is not issued by them as well? How can I possibly be suspected of entering the destination country illegally if I have passed check-in and they perfectly know that their destination counterparts at entry will check me themselves?
i can understand an answer of the kind of "it's their country and they can do whatever they want" but either there are reasons behind it or it should be said that their laws, despite being legit for sovreignity principle, are just stupid and meaningless.
Does it happen in most countries this thing?