Defederating only affects your instance and the other one, so it only affects users on your instance and on the other one (who cannot see content on your own). To get around it requires another account on an instance that has not defederated.
Even aside from differences in opinions (blocking literal nazi instances for example) there are very valid reasons for defederating. Some places do not control spam bots, for example, or an instance may also be a bot farm itself.
Most instances default to federating with any instance they find out about, but there is also the ability to make this a whitelist instead and only federate with specific servers.
Defederating only affects your instance and the other one, so it only affects users on your instance and on the other one (who cannot see content on your own). To get around it requires another account on an instance that has not defederated.
Even aside from differences in opinions (blocking literal nazi instances for example) there are very valid reasons for defederating. Some places do not control spam bots, for example, or an instance may also be a bot farm itself.
Most instances default to federating with any instance they find out about, but there is also the ability to make this a whitelist instead and only federate with specific servers.