Question: Use of parentheses in function application

Why no parentheses around "%" are needed here

cos^2 + sin^2 = 1;
%(x);
                           2      2    
                        cos  + sin  = 1

                           2         2    
                     cos(x)  + sin(x)  = 1

as they are required here (to group terms)
 

(cos^2 + sin^2 = 1)(x);
                           2         2    
                     cos(x)  + sin(x)  = 1

Is it because % (?ditto) is an operator or because of an evaluation rule?

I know the opposite: Maple treats (sin)(x) the same as sin(x), i.e. removes the parentheses.  
I also know that Maple has special evaluation rules for procedures (see ?last_name_eval) but the
"operator expression" cos^2 + sin^2 = 1 inside the parentheses is of type equation and not of type function/procedure.

I assume that % reevaluates a previously computed result stored in the kernel and not in the user interface but maybe I am wrong here. If equation labels are used instead of %, also no parentheses are needed, but in this case the equation label refers to output stored at the level of the user interface.