Existing paying customers? Why would you spend money to acquire customers that you've already acquired. You had your reasons for signing up for DO, and apparently, the lack of a $10 credit wasn't one of them. And we all know this is a flimsy firewall to breech anyway - a different e-mail address that - if you're really feeling saucy - you could have delivered to an SMTP daemon on your existing droplet.
Promotions are by definition a form of publicity or advertisement, and if you're already a customer, you're already a customer, know what I mean? Why advertise "hey, check us out" if you've already checked them out and signed on the dotted line.
If you feel really, really, really burned by this, do what other cost-conscious consumers do, and whenever you see a box "promo code" on a signup form, Google "example.com promo code" (substituting the actual domain name, obviously) and see what you can find.
I do not understand, even a little bit, the amount of outrage over the fact that you want a company to spend advertising money (that's what a promotion is budgeted to) to advertise to an existing customer. Not unless you feel that DO isn't worth it already, that is, and in that case, what the hell is $10 going to do to change your mind?