Convert all CSV files in a given directory to JSON using Python
Looks good to me. It's a perfectly sensible approach.

There's just one line I'm going to criticize:

    csvfile = os.path.splitext(filename)[0]


Picking off element zero is fine, no need to change it. Other idiomatic approaches would be name, ext = ..., or an explicit discard of name, _ = ....

What I really do advocate changing is the identifier csvfile, since the .csv extension has been stripped from it. Better to simply call it name. Then an expression like name + '.csv' can be clearly seen as a csvfile expression.

You could elide the jsonfile assignment, if you like, and similarly rows, replacing each reference with the expression.