Question on a rewrite rule

Hello

After installation of Letsencrypt free SSL, it adds a rule into apache's 
config file:

RewriteCond %{SERVER_NAME} =sample.com
RewriteRule ^ https://%{SERVER_NAME}%{REQUEST_URI} [END,NE,R=permanent]


I know it means when SERVER_NAME is sample.com, it rewrites the plain 
HTTP request to HTTPS request.

My question is, is this not a dead loop?

For HTTPS request, the SERVER_NAME is also matched, so it rewrites again 
and again...

My guess it's a dead rewrite loop, but it actually works fine.

I am not sure about this, please help.

Thanks.