Maven Java Version Configuration ignored by Eclipse/Idea

I have:

<build>
  <pluginManagement>
     <plugins>
        <plugin>
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-compiler-plugin</artifactId>
           <version>3.1</version>
           <configuration>
              <source>1.6</source>
              <target>1.6</target>
           </configuration>
        </plugin>
     </plugins>
  </pluginManagement>
</build>

Yet I have no problem declaring:

public enum DirectoryWatchService {

    INSTANCE;

    private java.util.Optional<String> test;
    private java.nio.file.Files files;
}

Eclipse doesn't bother. IntelliJ neither. Even Maven does not bother. I can even do a mvn clean package. Builds the darn thing without any warning whatsoever.