The premium version finds more errors than the basic version, including the incorrect verb form in your example sentence. If you want to improve the free version, you're welcome to contribute to the Open Source version at https://github.com/languagetool-org/languagetool

Update: There's a free version you can contribute to and a premium version that people work on full-time. We pay these people by the revenue we get from the premium version. If there was no premium version, we couldn't pay these people. You will surely understand that.