Who would you expect to receive a message from? Presumably this happens more often than you think, and the company protocol is to get legal to deal with it. I think you've misinterpreted this as in any way "aggressive" - this is just a company trying to tie things up as efficiently as possible. Sure, you'd like an informal response, but when a company is operating at the scale of Mailchimp, that doesn't seem realistic.

I was also quite impressed by the tone of the message from Mailchimp legal - probably the clearest, most polite takedown request I've seen.