the response driving a male to "protect" an abused female is itself a controlling response. men will step in under the hopes that they can take control of a submissive female. there's no such drive to protect a male (that is competition) from a dominant female.
the error underlying the experiment is the assumption that people are stepping in to "help" out of altruism, rather than to further their own aims.
the guy they picked was a little scrawny - small enough that he could be easily beat down and "his" girl could be "stolen" from him with little effort. do it again with a bigger, more alpha dude and you'll see a different response.
http://www.youtube.com/watch?v=cywQhs_6iC4