There is no a priori reason to exclude smaller parties

No one could object that the winner of an election should get some power but we might argue against that person getting all the power. Part of the problem when arguing against the first-past-the-post system is that this term doesn't include a description of the problem, namely that the winner gets all of the power not that the winner gets some of the power. We would not object to the winner getting some power but we might argue for some of the other candidates also getting power. An alternate term for this system sometimes used is winner-takes-all and this gets closer to the nature of the problem. We might propose a third title; first-takes-all, which might get closer to highlighting the problem.

It is against the principles of democracy for smaller, minority candidates and parties to be ignored by the system so then fptp is anti-democratic which makes no sense in a voting system, which we would expect to be democratic. No state has the right to oppose democracy. We might argue that democracy itself is not a right, and there is some truth to this but then for the state to replace democracy with something else is never a right, and the state in doing this is acting criminally. To replace proportional representation with anarchy is not a crime but to replace it with something which is not proportional is a crime.

The problem with first-past-the-post is not that the plurality winner attains power, the problem is that only this person gets power. In a democracy there is no justification in excluding less-popular parties from the legislative process, in fact it could be argued that the reason to have democracy to begin with is so that these minority interests and concerns are respected instead of being ignored as they would otherwise.