Beause it is. The greater heat absorbed by lower albedo melts more ice and snow. lowering the albedo even more as bare (darker) spots develope.
Also often there develope pools of water in low spots and the albedo there is lowered too, compared to the ice under the bottom of the pool. Again, I challenge you to find any Postive Feedback as strong as the one happening NOW in Greenland's ice sheet.
While this is likely true for Greenland, it's not at all clear it's true for sea ice.
Brewster's angle (53º) is the angle where about half of light is reflected off the surface of water. This means that above 53º latitude, naively, the albedo of water is over 0.5. (It is far more complicated than that of course -- which is my point.) Since ice and snow's albedo ranges from 0.9 to 0.3, it is not clear water's albedo is lower than ice in the arctic -- even in the summer. (In the winter the albedo is irrelevant because there is effectively no incident sunlight.)
Meanwhile, the arctic area can still "see" the night sky at a 90º angle. Its emissivity remains and the area continues to emit longwave infrared radiation. The limitting factor on this is likely the ∆T4between the ground/sea and the night sky. Raising the temperature in the region raises the ∆T and gets that 4th power emission bonus.
I want to make it clear I am not claiming this melting is a good thing. I'm just claiming it has been poorly studied. The arctic environment is probably the most fragile in the world. Given the large methane deposits in the tundra and oceans (clathrates) which even small temperature rises might release, there is cause for concern.
I can think of a number of things wrong with my model, the most obvious is that water (and arctic ice for that matter) is not flat. Waves will create a chop effect limiting the albedo gain due to the Brewster angle. The Brewster angle will change with the seasonal axial tilt. The atmosphere above the ice will have a large effect since waves incident at a low angle will travel through much more air. These conundrums are just off the top of my head. My point is that this is a complex subject and any positive feedback loop is at best not obvious.
But the idea that the albedo is of great concern in a region getting little sunlight seems oddly immune to logic. Global warming is a big enough problem without adherents practicing bad science and settling for confirming their bias.