People generally do what is expected of them, and everything in law enforcement's demeanor said, "We expect there to be no trouble and for you to keep having a good time while doing your thing." They weren't tense, so nobody else was either.
It also helps when the demonstrators themselves help keep the more obnoxious among them under control. (This is one comparison where I think the Madison Sit-Ins and many Tea Party demonstrations have something in common.) "Mob" mentality can spread easily if reasonable people allow it, and it is a good thing to see real people participate in representative government processes without the violence that has often plagued our nation's history.
But let's get back to the police & law enforcement, and how their tactical stances can help keep a demonstration peaceful. Demonstrations are usually high-emotion events, and if you inject more tension or emotion into such a situation where it is unnecessary, you're adding artificial risks to the situation. There is a huge difference between preparing for confrontation and expecting confrontation, and tactical postures tend to reflect this.
But so does human psychology. If you are expecting confrontation with a crowd, your standard operating proceedures and situation briefs are very different. One person's misbehavior under such circumstances can be misconstrued as crowd misbehavior or an accident can be mistaken for intentional provocation. Multiply that with the general confusion that exists in the coordination of naturally chaotic scenarios, and you have an atmosphere ripe for overreaction, momentum, and panic.
On the other hand, if your organization is prepared for confrontation, but not expecting it, your membership will be more confident in a tactical response because they have options. Your standard operating proceedure focuses on investigating individual misbehavior, accidents, and mistakes; the chances of overreaction and panic, though still existant, are much less because there are stages of readiness where someone can exert command of the situation and calm things down.
The Madison Sit-Ins are just the most recent example of such a dynamic. Here, it appears that the law enforcement and the demonstrators have an almost professional, event-based rapport; communication in such situations is enormously advantageous in a tactical environment. The demonstrators have been free to focus their grievances at their elected representative (which is how representative government works with the right to assemble); law enforcement has been able to expect crowd support and self-policing in keeping crowd control (the only real way to ensure majority peaceful demonstrations), while focusing on accidents, individual misbehaviors, and the isolated incidents of medical emergencies any large gathering is sure to experience.
Changing the tactical posture of the law enforcement to keep demonstrators out of the usually public-accessible capital building places law enforcement agents in-between the demonstrators and the individuals to whom their grievances are directed.
It is also places law-enforcement in a more difficult situation, tactically, as they must now expend resources and planning blockading certain spaces. They must attempt to allow certain individuals through the doors, while keeping others out - and this presents its own new set of provocative issues. Also, by focusing their efforts on certain locations, they are not free to move about the crowd and investigate the accidents, medical emergencies, and individual misbehaviors as they were before. This creates more confusion, less communication, and increases the tension level. That's not conducive to the continuation of peaceful demonstrations.
Make no mistake, this is an intentional decision on the part of those in control to provoke the demonstrators, and redirect their grievances against law enforcement; law enforcement has been placed in-between the demonstrators and their goal, and a more adversarial posture has been assumed even before the riot gear makes its appearance. The plan is to get the demonstrators to go away or do something that can be used as an excuse to clear the area in a crackdown (and demonize the event forever).
Hopefully, the demonstrators will prove their peaceful nature in the face of this deliberate increase of tension on the part of those in charge.