import - Why does ReadString complain about the stream not being opened in binary format?

This works fine:

stream = OpenRead["ExampleData/strings"];

ReadString[stream, EndOfFile]
(* "Here is text. 
And more text.
" *)

Close[stream]
(* "ExampleData/strings" *)

But if I use EndOfBuffer instead of EndOfFile, Mathematica issues an error message (despite returning the expected result).

stream = OpenRead["ExampleData/strings"];

ReadString[stream, EndOfBuffer]

During evaluation of BinaryReadList::bfmt: The stream InputStream[Name: strings, Unique ID: 5] has been opened with BinaryFormat -> False and cannot be used with binary data.

(* "Here is text. 
And more text.
" *)


Close[stream]
(* "ExampleData/strings" *)

Why does this happen? Is this a bug? This error is normally shown when binary read functions (such as BinaryRead) are used on streams that have BinaryFormat -> False. But ReadString is not such a function. It is designed to read textual data, with newlines interpreted.

---

Why am I trying to use EndOfBuffer? I am looking for a solution for reading form Mathematica streams in C++ code. One piece of the puzzle is reading only a limited amount of data (that fits into memory) with good performance. I thought that instead of trying to explicitly control the size of the data returned, perhaps I should read as much as Mathematica itself keeps in its buffer. Then running out of memory shouldn't be a problem.