Limiting to only one restaurant obviously reduces the amount of customers you can have. More potential customers means more potential profit. McDonalds isn't the only popular fast-food restaurant chain, there are loads more that get plenty of customers regularly too. The overhead cost would easily level out in the long run.
Keurig should try to partner with as many restaurants as they can to make their brand grow. The more places they serve drinks at the more people will like the products and want to continue to buy it out and also buy the Keurig for their house. I think it would only benefit them to expand even more!
By partnering with McDonald's, Keurig is making a important first step in the restaurant market; as well boosting the market potential for both Keurig and McDonald's new McCafe brand. However, if Keurig wants to maintain tight control over the market and limit or prevent future competition arising they need to expand their market to as many restaurant venues as possible. Even if those chains are in direct competition with McDonald's. Premium coffee at a discounted price is a growing market, and Keurig has a chance to seize; if they act quickly and make their name the name in the market.
While McDonald's is certainly a premier supplier, their coffee is not in any way iconic. Other restaurants, such as Dunkin Donuts, have a much stronger reputation for their coffee. If Keurig were to limit themselves to McDonald's, which is not perceived as a leader in that space, the company would be creating a tremendous opportunity for a competitor to co-brand with a rival coffee supplier and steal significant market share from them.