Unexpected result when reading Cell with comment-lines by NotebookRead

Question

When considering this question about re-formatting Cell (also this one), I encountered an issue which confuses me. I'll let the code itself speak:

Consider the following simple Cell expression, which contains a comment line:

cellexpr = 
  Cell[BoxData[{RowBox[{RowBox[{"a", ";"}], "\[IndentingNewLine]", 
       RowBox[{"(*", RowBox[{"b", ";"}], "*)"}]}], 
     "\[IndentingNewLine]", "c"}], "Input"];

Write it to a Notebook:

testnb = NotebookCreate[];
NotebookWrite[testnb, cellexpr];

It looks like the following, which is what one would expect:

Now read the Cell expression back from testnb:

SelectionMove[testnb, Before, Notebook];
SelectionMove[testnb, Next, Cell];
NotebookRead[testnb]

Note the first RowBox, it has a structure as RowBox[ item1, item2, ...], which is ill-posed since the first argument to RowBox must be a list. And the obtained Cell expression is therefor ill-posed, which can be seen by CellPrint-ing it:

I noticed that if the comment-line part is removed (e.g. by deleting "(*" and "*)" in cellexpr), everything works fine. And for the example in this question, the second comment is correctly obtained:

My question:

Did I miss something? Or is this a bug?

My system: Mathematica 9.0.0.0 on Windows 7 32-bit