It's unprofessional to write a blog post about your account getting deleted without notice? Mailchimp is a business, and the relationship between a person and a business is not friendship. When they shut down an account without notice blogging about it (in a non-accusatory manner no less) is completely reasonable. This isn't a smear job.

Talking in public about the manner in which businesses conduct themselves is a good thing. It benefits businesses with good customer service and hurts those with bad service. More transparency is good for consumers and for businesses alike.