For reasons of economics.
As an analogy, if you're a one-person developer, your problem is usually that you don't have enough users. You can participate in forum discussions with users of your software, fix bugs within days, send a personalized thank you e-mail for every purchase, all to provide exceptional level of service to grow word of mouth and get more customers. When you get to the size of Microsoft, that won't work anymore. Your expensive devs have to separated from users by layer of product management, your support calls cost you serious money and one troublesome buyer can cost you much more than he paid for in software etc.
Google finds itself in the same boat with AdSense. I don't know the exact scale of AdSense but it must be in millions of publishers and thousands of advertisers. Even a small number of people insisting you give them special consideration or explain in detail what exactly did they do wrong will burn incredible amount of support time and money.
It's clear why he was banned from AdSense and yet he wrote an epic post detailing his experience and inventing multiple reasons for why it wasn't so bad. Do you think that if someone at Google sent him a personal note explaining why he was banned, he would just say "oh, I get it now, you were so totally justified in terminating my account". No, he would keep badgering Google until they finally gave up responding and he would end up writing the same epic post, this time quoting extensively his correspondence with heartless Google employee who was not swayed by his obviously correct arguments.
Someone arguing with Google to not suspend their account has all the incentive to keep badgering Google until Google says "this is a final decision".
Google can't win this battle and they do the best anyone can do at that scale.