The same reason they don't restrict logis to filling all the equipment slot.

The same reason Heavies can carry light weapons.

The same reason any suit, with 2 weapon slots, can have 2 sidearms at the same time.

The same reason snipers can be tanked out heavies.

The same reason scouts can tank out their suits if they want.

It really bugs me that this game is advertised as a flexible FPS where they don't restrict what a player is allowed to do. Literally there is a video where CCP actually encouraged the use of heavies for sniping. And it's bad players that whine and moan, "Why can heavies use light weapons that's so OP!!!! WAAAAHH!!!! CCP PLZ NURF," it's ridiculous. If you didn't know what you were getting into when you started investing your time/money into this game then you should definitely move to another game(like COD or Battlefield where literally every character is the same).

Or if you want immersive gameplay that allows you to customize your character in any way you want, within reason, then stay and have fun knowing that you're not only better because you're a nerd with good gun game, but you're better because you're a nerd that's smart enough to find the best way to customize your character for your play, even if what you do is complete broken and OP.

In other words, it'll always be better to have an OP FOTM, than to force players to limit their playstyle by creating suit requirement/restrictions.