It's not that simple. They don't need to pay Motorola to sue others. If we're speaking legally here then your argument has no legs. If we're talking realistically then you have a point but the courts will think that's irrelevant and make a decision based on the merits of this case regardless of any hypocrisy by Apple either real or perceived.
Now, speaking from a real world perspective and not about law, I'd say that the Motorola case is a lot like the Ford case. I'm the case of Ford you really can't stop others from making 4-wheeled, passenger carrying, combustion engine cars. There was just no other way at the time. Same for "hand-held mobile telephone radios". If Apple gets sued for such a generic technology then every cell phone and walki-talkie maker not paying Motorola royalties should too. In the case of Apple v Samsung, they aren't arguing that others can't make a tablet. You can easily make tablets that are similar (because even I will admit that nowadays there really isn't much more you can do with tablet design) and people have without infringing. But the Samsung tablet is so close to the iPad that its almost indistinguishable unless you inspect it further. It's like if someone made a project management web app and called it "OperatingBase" and it had all the features of Basecamp, a website with the same colors, similar logo, and all the rest. Well 37Signals couldn't sue the other project management app developers (let's pretend they have design patents in this case) but they sure can try to stop the asshole who is very obviously trying to rip off their product and ride its coat tails by hoping consumers who don't know better will think its the same exact thing.
Now, speaking from a real world perspective and not about law, I'd say that the Motorola case is a lot like the Ford case. I'm the case of Ford you really can't stop others from making 4-wheeled, passenger carrying, combustion engine cars. There was just no other way at the time. Same for "hand-held mobile telephone radios". If Apple gets sued for such a generic technology then every cell phone and walki-talkie maker not paying Motorola royalties should too. In the case of Apple v Samsung, they aren't arguing that others can't make a tablet. You can easily make tablets that are similar (because even I will admit that nowadays there really isn't much more you can do with tablet design) and people have without infringing. But the Samsung tablet is so close to the iPad that its almost indistinguishable unless you inspect it further. It's like if someone made a project management web app and called it "OperatingBase" and it had all the features of Basecamp, a website with the same colors, similar logo, and all the rest. Well 37Signals couldn't sue the other project management app developers (let's pretend they have design patents in this case) but they sure can try to stop the asshole who is very obviously trying to rip off their product and ride its coat tails by hoping consumers who don't know better will think its the same exact thing.