At GitLab we try to cherish our contributors. Every month we celebrate a most valuable contributor https://about.gitlab.com/mvp/

We try to ship great new features in our open source version every month that you can use for free on your server and on GitLab.com

EDIT I can't comment for DigitalOcean, what follows is my personal opinion.

I think it is great that DigitalOcean is willing to both sponsor the Runners for GitLab.com and offer promotion codes. Although I understand with your wish for promotion codes for existing customers I also understand their decision to only apply this to new customers in order to make this cost effective.

I hope that as an existing DigitalOcean customer you enjoy the benefits of their cost-effective servers, quick boot time, great UX, and the templates and tutorials that they keep updating.