I can see the logic in it. It's more a question of what kind of experience are you looking for? (acknowledged on the BCU website, Canoe England section: "Sea kayaking for many, however, is a quiet, individual experience to be enjoyed quietly, away from the crowds.")
For me two is a crowd, but one I don't mind.
Partially you might consider the different cultural role of the canoe & kayak. Both are indigenous to North America and were developed for utilitarian purposes, basic transportation, hunting and fishing. People have been casual about their use here cause they've always been around. In the past few years the police have started doing on-water patrols and handing out tickets to paddlers and powerboaters alike for everything from lack of lifejackets (understandable) to lack of a whistle (a bit over the top), as well as the perennial favourite "open liquor". However no one has mandated paddling in groups so I put the safety equipment handy and do my thing.
I saw a TV show aptly titled "We Should Be Dead" about two incredibly inept sea kayakers who got into trouble in Puget Sound, cause they did EVERYTHING wrong due to macho stupidity. One was experienced, the other one lied and said he was but had never paddled before. My favourite part: Only one had a spray skirt that fit, but they agreed that it would not be used because they didn't both have one...then they tried to cross three miles of open water and surprise, the rookie paddler couldn't handle the boat (didn't know there were foot pedals to help steer) and took on a lot of water. The other paddler was way ahead already, saw his friend in trouble behind, and decided to forge ahead to land and get help. Meanwhile Stupidhead #2 just saw his buddy abandoning him and had no idea why...he drifted around all night till he managed to land on an island by a lighthouse, and the search & rescue picked him up in the morning.
Now if there had been three of them, #3 could have stayed with Stupidhead #2 and they could have drifted around together...but by far the better plan would be not to have gone out until they were properly equipped for the conditions, (they launched just before dark too!!!) and each been honest about skill level. Paddling isn't rocket science and someone who is a complete novice but has the requisite physical strength can get through pretty heavy water with a little coaching IF THE BOAT IS APPROPRIATE FOR THE WATERS AND PROPERLY SET UP.
Oh yeah, and I take a cell phone, also not mandatory. Never had to make a distress call, but it's fun to bug my (few) friends by calling them up and saying "guess where I am!"