Phoenix is in an area of Permafrost. The "normal" rules don't apply here. That's the trouble with "common sense" it's only Earth-normal common sense. It doesn't translate directly to other planets.

What's happening on Mars is that there is a thin skin of relatively loose and ice free dirt. Every summer, that skin warms up and bakes out any ice. Every winter, some new ice (mostly CO2, but some H2O) forms on the surface, and every spring that all boils off.

meanwhile, at depth, is another world. Permanently frozen dirt lies everywhere in the northern plains of Mars. At a depth ranging from a few cm to half a metre or more, the top of the "ice" is the top of a permanently* frozen zone. Dirty ice, icy dirt, segregated lenses of ice and dirt. This is the upper crust of Mars.

* with climate change, conditions do vary, so the top of the frozen zone may rise and fall a few cm or tens of cm on a 50,000 to 50 million year timescale.

Your question? Why doesn't it blow away or pile up?

1) it does.

2) it can't

A bit of both.

a) The ground is very flat, so wind can't really get a grip on anything.

b) The dust is weakly ice-cemented and also slightly "stuck" together like all the adhesive surface soils of Mars - Notice how a clump of the soil from the middle of the "Yeti footprint" stuck to the back of the digging scoop. remember how the soil forms clods in the treads of the rovers. It's a bit like flour on Earth. If you pack it together, it sticks in a clump. If you leave it in a pile for a few thousand years, it sticks together.

c) The system is self-adjusting. if some dust blows away, the ice underneath is exposed to more temperature variation, and ablates over the next few summers. If dust piles up, the ice grows into the space. So there will essentailly always be a few cm of loose dusty regolith on the top of a frozen zone. Even the dunes in the circumpolar sand sea will have frozen cores, and yet they will migrate with time, because the ice is active.