Capitalism, fine. Competitiveness, fine. Postmark is not a company afraid of customers or competition. But they believe in competing in a classy, ethical way. Most importantly, their competitive advantages are based on facts, things they can control, things their competitors could control but don't.
How can anyone disagree with that?
But sure, let's look at it from a ruthless, ethics-free standpoint:
What if you do run attack ads, what if you do kick the competitor when they have a freak event that they did not cause and could not have prevented -- an enormous DDoSing campaign by an organized group -- and which could happen to you, too?
What if you lure customers away with these attack ads?
What will those customers do when you have a freak event?
No service has perfect uptime. DDoSing can happen to anyone.
So even if you have the ethics of a rutabaga, all you get for your efforts are customers which do not tolerate the occasional freak event. What do you think they're going to do when you have something you have no control over?
Your marketing will have created the results that you deserve.
Business doesn't exist in a vacuum. You do not "win customers for life." Customers are not all equal (far from it). This is a long game. Being classy, being honest, is the best way to create a business you -- and your customers -- can live with for a long time.