I think you have to have the relevant factory built:- IE you need a hand of nod built in order to build Shredders, and a war factory built in order to build laser turrets and so on.
I could be totally and utterly wrong.... but I do remember one RTS (I haven't played many) that definitely did it this way.
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If I tried to snipe the ground I was standing on I'd still miss..... 
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