It's an interesting point, and of course there are many instances of boycotting in everday life - action groups often call for specific companies, or countries even, to be boycotted - usually for humanitarian or environmental reasons. Usually the boycott request is backed up by concrete evidence. This is slightly different from someone who posts in our forum along the lines of:-
"I bought a monitor from ScreensRus and it went wrong on the first day. I phoned them and they couldn't have cared less. They obviously wanted to avoid complying with consumer law, They're a bunch of thieving cowboys, and they took my money under false pretences, don't touch them with a ten foot pole"
That is a potentially libellous statement about a specific retailer, and is published where tens of thousands of people will see it.
As Forum Editor I have a duty to ensure that the magazine isn't exposed (even inadvertantly) to the possibility of a libel action. That possibility may be slight, or it may not, it depends entirely on specific circumstances. What is certain is that there have been successful libel actions against the publishers of web forums, and they have been brought on the basis of what was written by forum contributors.
If I'm going to err in this it's going to be on the side of caution - one big award of damages is one too many - and that's why I operate the "no exhortation to boycott" rule. Quite apart from anything else, I'm sure you'll agree that it's ludicrous to expect thousands of people to avoid a supplier because one person had a bad shopping experience. If that was the way to proceeed we would all have run out of available suppliers years ago.
Let common sense be your guide when posting for public consumption, and all will be well.