Most likely the non-dimmable instruction is for legal purposes. The fixture manufacturer has no control over the lamps that the end user might install. So to establish a defense for the manufacturer and the retailer, the non-dimmable designation serves that purpose.
I know of no technical reason that a dimmer control cannot be used PROVIDED that the lamps used ARE suitable for use with that dimmer.
CFLs should be OK if not on a dimmer. Most CFLs are not suitable for dimmers and those that can be dimmed may require only specific compatible dimmers. If the lamps are enclosed, better stay away from the CFL.