R&D and payback
A couple of hundred million isn't all that much in the grand scheme of things. Consider that in principle LiquidMetal can be formed more like plastic or sheet metal, yet produces a result probably superior to the CNC machined unibody cases, and $200 million isn't a huge investment. The reduction in unit cost of the product, and yet a serious improvement in that product would, with Apple's volumes, mean the $200M could be amortised almost instantly. From the linked to article:
"Therefore, I expect Apple to use this technology in a breakthrough product. Such product will likely bring an innovative user interface and industrial design together, and will also be very difficult to copy or duplicate with other material technologies."
Think unibody design, but on steroids, and an exclusive right to produce them. Very Apple. Also very forward thinking, with a long game plan. Something else very Apple.
Note that Apple is a licensee, they don't actually sue other users of LiquidMetal, the LiquidMetal company does. If LiquidMetal decide to breach the contract with Apple, and give the tech to someone else, then LiquidMetal get sued by Apple.