there are several techniques for loom knitting..
one uses both sides of the loom
this creates a double sided fabric, that is thick, plush and warm.
It won't curl
you can also use a circular loom or half the pins on a straight loom to do a form of stocking knit stitch.. and this (like all sorts of stocking knit stitch!) will curl.
It is possible (*but a bit tricky!) to do a purl stitch on a loom.
a few purls (ribbing (alternate column of knits and purls) will help resolve the curling.. so will seed stitch (a checkerboard pattern of knit & purls)
but stocking knit curls!
a scarf knit in the round (on a circular loom) is double thick (a flattened tube) and this won't curl.. (but many looms are designed for chunky weigh 'wool'/yarn and making a doubles scarf (either flat loom doubled) or a circular one (flattened tube) is going to make a thick (and expensive) scarf.