Because you still did something unethical? Just because one company did it, doesn't mean it's ok for multiple companies to do it.
Likewise, if it excuses it, then surely I could just set up some shell corporations, make a bogus price-fixing deal like Apple and publishers, and then have my real company do the same deal "because that company got the terms first" and then when it comes time for investigations, I just throw my hands up and say well we just had to compete with that other companies. They did it first, after all, and we're just trying to compete.