Why is an extra semicolon not allowed after the return statement, when it is allowed for other statements?

Why multiple semicolon is not allowed after the return statement, when it is allowed for all other statement?

Simply because when you have a statement like

System.out.println();;

This means you have two statements, one is System.out.println(); and other statement is after the first semi colon, it is empty and that's allowed BUT you can't have any empty statement or any other statement after the return statement because it will never execute, in other words, its unreachable statement and you can't have unreachable statements in your code.

Same thing happens in this code too

if(a == b)
    System.out.println();;
else
    System.out.println();

that's because, when you have an else statement, statement just before it should be if statement which is not the case in above code snippet because statement just before else statement is an empty statement which is not allowed.

If you have parenthesis after the if statement like

if(a == b) {
    System.out.println();;
}
else
   System.out.println();

you will get no errors because now empty statement is inside an if block and the statement just before else is if statement and not the empty statement which was the case when you had no parenthesis after if statement

Your code:

if (a == b)
    System.out.println();;
else
    System.out.println();

is equivalent to

if (a == b) System.out.println();
;
else System.out.println();

And you can't use an else if the preceding statement is not an if.