The short answer is no. Although we have never heard of Askimet contacting anyone to complain or ask for payment, their only free option is for a personal blog. They don’t include a definition of personal use, but if you are making money from your WordPress site you can assume you are breaking their terms and conditions. They don’t mention any specific penalties for using a free API key for commercial use, although they reserve the right to terminate service without reason. Here is one section that applies to your registration.
By registering with Automattic, you represent and warrant that the information you provide to Automattic in connection with any registration process is true and accurate, and that you will promptly notify Automattic if any of that information changes.
Akimet is made by Automattic which is the company that created WordPress.com. To register as a business the cost of Askimet is $5 per month. With such a focus on free open source software, a plugin that requires an API key and often a paid version seems out of place.
What to do about it
Although $5/per month isn’t much its annoying and unneccesary. Its another bill to pay and another password to remember. Also, Askimet isn’t fighting to stop spam, they are fighting to block it using their software. The more spam that hits blogs that are NOT using Askimet, the better for them. Read our post on how to fight WordPress Spam without Askimet and use truly free plugins instead.