As Jude said, it's not illegal. However, I think it's a gimick. A lighter stick seems to let you get more cue speed on the break shot.
This is true for the most part, but isn't the only contributing factor to an effective smash break.
When your goal is to get the cue ball traveling as fast as you possibly can, you are actually trying to transfer as much linear momentum as you can on that cue ball. How effective you are depends on two things - how much linear momentum you can create, and how much of that momentum gets transferred when you strike the cue ball.
Since momentum is mass times velocity, it's obvious that you can change momentum by changing either of the two factors. But these factors are not independent. You are applying force to create momentum, and the force you are capable of applying is pretty much constant. We all know the physics principal that force = mass X acceleration, so increasing mass will decrease acceleration, thereby decreasing velocity. All other things remaining equal, an increase in mass will decrease velocity by a corresponding amount, and momentum will remain unchanged.
But all other things don't remain equal, and this is where the weight of the cue can make a difference. For a lot of people, a heavier cue means they have to expend more energy just to move the cue in a straight line. This extra requirement reduces the amount of energy they can put into acceleration, and in the end they are able to produce less momentum than they could if they were using a lighter cue. People who are physically stronger and have no problem handling the heavier cue don't tend to have this problem.
The other factor is much more important in an effective smash break. How much of the linear momentum is transferred to the cue ball? This all depends on the elasticity of the collision between the cue and the cue ball and the squareness of the hit. Elasticity depends on the materials involved, and is why people prefer the phenolic tip when breaking. It doesn't absorb as much energy during the collision, so the collision is more elastic and more momentum (speed) is transferred to the cue ball.
The material and construction of the cue itself also affects elasticity. Imagine that you have a giant sponge, and you carve a cue-shaped piece from it. Add enough water to the sponge to make its weight equal that of your cue. Now try to break with it. Not gonna work, is it? The sponge absorbs most of the energy of the collision and very little momentum is transferred to the cue ball. Similarly, if a cue vibrates upon contact, it absorbs energy and reduces the elasticity of the collision.
The squareness of the hit is a big player in transfer of linear momentum. If you don't strike the cue ball perfectly square, part of the linear momentum is converted into angular momentum (spin) during the collision, and the cue ball won't travel as fast. An off-center hit is also a glancing blow, meaning the cue will retain more of it's linear momentum and less will be transferred to the cue ball. A heavier cue that is harder to control may very well affect your ability to contact the cue ball squarely.
There's probably a lot more detail to it, but these are the general physics of the break. To be most effective with the smash break, you should break with the heaviest cue that you can easily control, made of the hardest materials available and constructed in such a manner as to reduce vibration.