Sunday sailings

I found it quite interesting to hear that the possibility of Sunday sailings will be discussed at a Calmac Board meeting on Wednesday. What makes it even more interesting is the demographics of the opposing camps.

Thousands are rumoured to have signed a petition, spearheaded by the Lord's Day Observance Society, opposing Sunday sailings. A couple of dozen have contacted Calmac to ask the ferry operators for a Sunday sailing.

I am aware of the sensitivities involved, and that those commenting on balance appear to be opposed. However, I do have to point out that legally speaking, the opponents haven't got a leg to stand on. How come?

1) Ferries already ply to and from and within the Southern Isles on Sunday, so why not from Lewis?

2) Planes already fly in an out of Stornoway Airport on Sunday, so why shouldn't ferries sail out of Stornoway harbour?

Am awaiting outcome of said Board meeting with bated breath.

EDIT
Calmac have postponed this discussion for 2 months, in order that it does not interfere with a debate about Road-Equivalent Tarriff. This would mean that ferry travel would cost the same as travelling the same distance by road. The Stornoway to Ullapool ferry is one of the routes under consideration for this.