What's wrong in this templatized operator overload ?

[MobPassenger via Digitalmars-d-learn]

On Thursday, 22 October 2015 at 04:01:16 UTC, Mike Parker wrote:
> On Thursday, 22 October 2015 at 03:19:49 UTC, MobPassenger 
> wrote:
>> code:
>> ---
>> struct Foo
>> {
>>     bool opIn_r(T)(T t){return false;}
>> }
>>
>
> This needs to be marked with const:
>
> struct Foo
> {
>     bool opIn_r(T)(T t) const {return false;}
> }

what's the rationale ? what's guaranteed by the qualifier that's 
not already true without const ?