Apple and Microsoft are notorious software patent aggressors who consider patent protection racket an acceptable "business practice". Google on the other hand tries to avoid anything of that sort, and uses patents only for defense against patent aggressors. At least Google used to up until latest Motorola acquisition which mixed their policy up. Didn't really look into what Samsung is doing in detail, but they usually just fight back (for example to Apple) as defense, and don't act as aggressors.

Yahoo didn't engage in software patent aggression before. So whatever the case, acting as a software patent aggressor shows deterioration of company ethics and usually signals that they can't compete on merit.