Why can't function expecting immutable arg take mutable input?

[Shriramana Sharma via Digitalmars-d-learn]

Hello. I still haven't wrapped my mind around the const/immutable thing yet 
and am still stuck in C/C++ mode. :-(

A function that takes mutable arguments cannot be called with immutable 
input at the call site since it does not promise to *not* mutate the input. 
That's of course clear.

Why can't a function that takes an immutable argument be called with a 
mutable input at the call site?

IOW, why isn't mutable implicitly convertible to immutable?

I just finished writing a string processing module which calls multiple 
subroutines, and all of them carrying arguments with type `string` viz. 
`immutable(char)[]` IIUC, and I tried to pass it something which came from 
File.byLine(), then got the error:

function textattr.applyTextAttr (string text) is not callable using argument 
types (char[])

I understand that const can refer to either mutable or immutable, so does 
this mean I should replace all occurrences of `string` in arguments and 
return values of functions by `const(char)[]`?

-- 
Shriramana Sharma, Penguin #395953