We've been offering our public LanguageTool HTTP API for quite some years and it's used by several websites. However, some of these websites don't give credit to LanguageTool at all and I think that's not fair, especially for those kinds of websites that basically just copy LT functionality.
Thus, I've changed the rules under which the public HTTP API can be used: we now expect users who use the API to power a public website to set a link back to https://languagetool.org. I think that's not too much to ask for. In the next few weeks, I'll contact the sites that don't link back yet and ask for a link.