Why can you buy a keyword but not display it?

Recently I started working on a project dealing with Guns and Firearms and noticed something that does not make any sense what so ever. You are allowed to purchase the keywords "Firearms" and "Guns" however you are not allowed to input them into your ad displaying to the searcher that this is the correct ad to click on.
This pertains to the quality ad score which determines cost per click and position. However a simple solution to this problem. Using { } brackets containing "Keyword" will display whatever keyword that was typed in automatically upon the search. For instance, if your keyword you are purchasing was "black nike shoes" your ad may say :

Black Nike Shoes
Browse Nike, Reebok, Adidas.
$25 Purchase. Free Shipping.
www.WillsNikeShoes.com

But you also may be purchasing "Blue Nike Shoes" "Red Nike Shoes" "Hightop Nike Shoes", instead of making an ad for each keyword to display properly within the ad, use {keyword} to display the keyword (when character space is available) to the searcher to "call to action" or click on your ad. This will also allow you to display those words in your ad that they would not allow you to input previously, be aware if the words are against all of their rules you may have ad turned off or worse, suspended. This tactic is a time saver and very effective to convey your message to the potential customer.