Tunisian crochet in the simple stitch will curl. You can use combinations to stop that curl making an edging as you go if you are doing something like an afghan. You can use the simple stitch and combine it with the purl or reverse stitch.
Otherwise you have to block the piece or do some kind of other edging around the piece to stop the curl.
The reason it curls is just like the reason that a knitted stockinette stitch curls, all the stitches are to one side thus making the other side all one type of weave. This causes the piece to curl. When you use other tunisian stitches along with the tunisian simple stitch then this helps eliminate that curl by off setting the way the yarn lays with the new stitch.
If you are doing blocks in tunisian, when you attach most of the blocks the weight of the others will pull out much of the curl, but you will probably still have curl at the edges that you will need to fix.