Its clear that they use color to signify certain allignment aspects, which are completely in sinc with SW btw. The more psychologically aggressive colors, atleast in western culture, are offered only to the dark side allignment. I am happy they implimented this! Its a very obvious way to distinguish someones allignment from a distance. This keeps with the spirit of starwars anyway.
And your argument, though spoken eloquently, has a few loopholes in it. The restrictions stand more for principle rather then logic. We cannot loot purple lightsabers from low level mobs for the same reason we cannot loot the level 50 Jedi Guardian's gear from its corpse in PVP, and equip it on a different class. Logically, any character should be able to wear anything that fits... right? But there are restrictions, based on principle, in place. I for one am glad for this.
That said, I would like to see more saber colors, more gear options/models, and more customizations to individualize such as gear dye (like in WH). But please do not change the spirit of the saga just to suit ppl who want to dual wield red and blue sabers thanks bioware