Let's Encrypt is all about trust, and in order to protect our reputation and users we can't allow people to use our logo like this without our permission. In the case of site "badges," we are particularly concerned about any resemblance to "site seals," which promote bad behavior and misplace confidence. See this blog post for some of the reasons we have issues with site seals:
CertSimple.com
We're happy that people want to express support for Let's Encrypt (thank you!), but we need to make sure it's being done in the right way. We may provide images designed and approved by us for this purpose in the (near?) future.
If you have created a badge containing ISRG trademarks like the Let's Encrypt logo, or you are displaying such a badge on your site created by someone else, I have to ask that you remove it.