From discussion I had on IRC (in which some un-named DH developers were participating) I learned that our little "proof of concept" working code samples for automating the adding of subdomains with cURL and PHP (or something very similar) had been utilized by a few SEOtards to automate the adding of literally thousands of subdomains in a few accounts that had no other purpose than gaming the search engines (Gawd I detest these lowlifes!).
I was told that DreamHost's response to that was to have the "captcha" appear once "N" number of domains (or subdomains) had been added to an account, in order to discourage this wasteful practice.
I have no idea if other parts of the panel have similar controls now, but if I were the one programming it for the domain adding routines, I would have put such a failsafe everywhere else in the panel I thought it might be useful.
As is so often the case with these "marketing" vermin (SEOtards, SPAMtards, telemarketers, snake-oil salesmen, and their ilk.) their anti-social behavior (and the idiots that make it pay off for them) has resulted in the loss of a nice little bit of functionality the rest of might have responsibly enjoyed.
--rlparker