The purpose of trademark law is twofold: first, it is to aid the consumer in differentiating among competing products and second, it is to protect the producer's investment in reputation. The U.S. Supreme Court summed up this purpose nicely in 1995 in the case of Qualitex Co. v. Jacobson Products Co.:
"[T]rademark law, by preventing others from copying a source-identifying mark, 'reduce[s] the customer's cost's of shopping and making purchasing decisions,' for it quickly and easily assures a potential customer that … the item with this mark … is made by the same producer as other similarly marked items that he or she liked (or disliked) in the past. At the same time, the law helps assure a producer that it (and not an imitating competitor) will reap the financial, reputation related rewards associated with a desirable product."
Since foreclosure sales or carwashes (another case I recall) are not competitive with Toys "R" Us, the question is whether someone using "R Us" in a name is infringing on the reputation of Toys "R" Us. They are certainly playing off it, but are they doing harm? Similarly with any product or company called "Mc"something.