Actually, this is already checked.
But the issue remains if the MM-file contains multiple values per line. Then each value in a row seems to be interpreted as being on a row by itself. Thus the value is not interpreted as the next column value, but instead as a value for the next row.

Consider this file:

%%MatrixMarket matrix array real general
4 2
1 1
2 2
3 3
4 4

Then reading and writing it again one would expect (without deeper knowledge of the MM standard) the matrix entries in a single list using column major format:

%%MatrixMarket matrix array real general
4 2
1
2
3
4
1
2
3
4

But instead one gets

%%MatrixMarket matrix array real general
4 2
1
1
2
2
3
3
4
4