I would suspect that Mazda didn't want to drop the car as much as Racing Beat did because of ground clearance issues and to make it easier for the stealerships to do maintenance with out having to go and buy a but load a specialty equipment to deal with maybe one or two cars.
And who is to say Mazda didn't line their pockets on the deal. They get their OEM supplier to make some spirngs, Mazda gets a huge break on price because they already have a contract with them. Mazda probably didn't want to pay the extra money to have Racing Beat go to RB's supplier to make the springs. That would cost more. Time table factors were also present.
So they (Mazda) get a cheaper (cost wise) spring deal with springs that are basically the same as what Racing Beat gave them and slapped Racing Beat's name on the car. Hence, money for Mazda, pay less for the spring and charge the same as if it were made by Racing Beat's spring supplier.
Like I say, for all tense and purposes the MP3 has Racing Beat springs. The rates are proportionately the same. When the ride height is slightly different you don't need as stiff springs if the car sits relativley close to stock. Drop the car more, you need stiffer springs to deal with less suspension travel.
If you think we don't have RB springs it really doesn't matter. Go buy what ever you want. It's rediculous to argue over something this minute. It is my opinion however, if you own an MP3 and buy the springs that Racing Beat offers, you just wasted your money because you just bought something you already had.
If you don't have Racing Beat springs, they might be a worthy investment because they spent some serious development time making the Protege handle better. I can't say that for Eibach, H&R, etc...
AWR would be my next choice for a suspension set up.