I don't think they ever suggest that their way is the only way. There's nothing wrong with saying how you do something and making a case for it; you don't have to add a disclaimer to everything you write about doing business.
I think you have a valid point. The reason I partially disagree with it is that attitude plays a part here. 37signals comes off as though they think they need to tell people how to live; they love being up in your grill. The contents of their posts are often reasoned well enough, but their attitude says "you suck; let us tell you how to suck less."
I'm going to reply again just to say: bullshit. Compelling writing doesn't need anything to back it up. You either have a point or you don't. Acting like an asshole just means you don't have much of anything to say.
Actually it almost certainly wouldn't, unless you mean specifically for you.
They're constantly in the press, they generate conversations on sites like this, and they've spawned a legion of (number)(noun) imitators. It's one of the basic principles of copywriting and social media marketing that it pays to be bold and occasionally controversial.
I don't want to say that nice guys finish last, but certainly a big slice of the people at the "top" of the tech world aren't taking the middle ground. Pissing people off seems to correlate with success.