Not all surveys have to be universally "unbiased" - they just have to be reasonable for the core audience.

If Costco makes a survey for their members: "How happy are you with your Costco membership?", no one would whine about how the survey is biased against people who don't have a membership. If I made a survey saying: "How many hands do you have? 2 or less than 2?" This isn't biased against octopuses or something. I am assuming that normal humans with a maximum of two hands are answering.

It's not a survey for scientific research. It's a survey on democrats.org. Of course, they are making the core assumption that the audience is unhappy with Trump.