Because you got a better racquet than the cheap $40 WalMart racquet.
Did you buy the racquet because some pro uses it? If so, you would have bought that same racquet if that pro also used a "plasticy toy", right? So does it really matter if the pro uses a paintjob or not? You didn't buy it for the racquet itself but because it looks just like the one a pro uses, right?
A pro using the racquet means it must be able to stand up to some abuse, hence it's not a platicy toy. (Understand that I'm talking about consumer perception here which companies try to direct). Also, those who buy racquets because the pros use them do so for more than just looks. They don't understand your point about performance vs looks. It certainly is true that in the minds of many consumers if they get the equipment the pros use, they will be able to play like the pro. They may not be able to RUN around like the pro, but give them the racquet and they'll at least be able to hit like a pro. Absolutely consumers, especially kids, believe this.
There's also another aspect to this. Again, those who are buying racquets because the pros use them don't know enough about tennis to understand a game can't be "bought". But this goes beyond just "magically" being able to play. For example, when I first bought my Ti Radicals, I was just beginning in tennis. I absolutely bought those racquets because Agassi played with them and I wanted to emulate my game after him. For me, the racquet represented a measuring stick of sorts. If I was not playing like Agassi it's because I still had some work to do but I believed (*
*) that given enough time I COULD hit like him. In other words, all I had to do was elevate my skills to match the potential of the racquet. And of course, how these racquets are marketed, it's not possible to pick up a broom and beat some students.
My point is that most here know people who believe they can buy a game. With the paintjob racquets, the industry is taking advantage of that false belief and preying upon those who don't know better. That IS unethical IMO. Besides, kids do idolize their players and they do want the equipment their favorite player uses. It makes them feel connected to that player. I'd wager there is a lot of disappointment from them when they learn what they that what thought they were buying is essentially amounts to a fake.
Look at the flip side to all this. Suppose the companies didn't paintjob their racquets. Would they still sell the numbers of racquets that they do? My guess is that we'd see a lot less models offered from the likes of Head and Wilson and higher quality racquets too. I bet sales wouldn't be as good though. Maybe they shouldn't be.