Why do orbits of satellites orbiting at sufficient speed, far beyond the Earth's atmosphere, decay if there is no atmospheric drag? What drag is applied on the satellites at that altitude?
If satellites are truly far beyond the atmosphere, their orbits do not decay, except for very small perturbation effects like solar radiation pressure from the Sun or tidal forces from the Moon.
But the atmosphere doesn't sharply end where space begins (by convention 100km). The thermosphere still has a noticeable density up to 600km, causing significant drag and orbital decay for all low Earth orbits.
After that, the yet flimsier exosphere extends for another 10,000km, affecting medium Earth orbits.
The end of the atmosphere is a hundred times farther away than space.
The Wikipedia article on orbital decay lists a number of reasons, in addition to atmospheric drag:
An orbit can also decay by negative tidal acceleration when the orbiting body is large enough to raise a significant tidal bulge on the body it is orbiting and is either in a retrograde orbit or is below the synchronous orbit. This saps momentum from the orbiting body and transfers it to the primary's rotation, lowering the orbit's altitude.
Gravitational radiation is another mechanism of orbital decay. It is negligible for orbits of planets and planetary satellites (when considering their orbital motion on time scales of centuries, decades, and less), but is noticeable for systems of compact objects, as seen in observations of neutron star orbits. All orbiting bodies radiate gravitational energy, hence no orbit is infinitely stable.
Satellites using an electrodynamic tether, moving through the Earth's magnetic field, create drag force that could eventually deorbit the satellite.
While not a direct cause of orbital decay, uneven mass distributions (known as mascons) of the body being orbited can perturb orbits over time, and extreme distributions can cause orbits to be highly unstable. The resulting unstable orbit can mutate into an orbit where one of the direct causes of orbital decay can take place.