Actually its perfectly legal. In fact, mongoose even made a 3 speed, 1 in rear travel bmx race frame in the mid 90's.
The reason DJ and slalom bikes are allowed in BMX is because it is classified under the "cruiser" class, which means any bike 24in tire or bigger. Now they should never be racing 20in bikes as that is a different class.
And no industry, they don't get spanked. I have no issue tearing a guy on a top end bmx cruiser up on my urban/4x bike. Whole ordeal is 4x is basically dh bmx racing. It's completely changed since dual slalom. Your seeing more hardtails and stiffer suspension now making the bikes actually feel better than the bmx counterparts.
But yes, ABA and NBL allow them. Your screwed. its nothing about them being too cheap for a bmx bike. Its how the bikes feel. I absolutely hate how my bmx feels now since I race DH mtb and ride urban and 4x. My bmx bike has an awkward low down riding style and geometry, while my mountain bikes have a higher riding slung back feeling. Its easier to move around a 4x bike than a cruiser. All preferance. I actually like the fact more people are getting off bmx cruisers and onto 4x bikes. Its helping progress racing, and bringing a whole new style of riding and riders to the bmx scene.
And no benny, your thinking about "preloading" from a bmx stand point. You don't use suspension to "bump you up" you don't use it to help you jump. Its not like it used to be in the early 90's with those crap suspension forks they had. All the 4x forks do is soak up hard landings. Just like on my dh bike, I don't preload my 8in of travel when I jump, I just huck off the lip. Same on the 4x bike. Your not cheating by using suspension. In any case it will make them slower because each pedal gets cushioned by the forks taking away from pedaling efficiency, meaning you should have no problem railing them on a bmx bike.