paucity of traffic on this is itself an answer but I reckon topic ain't sexy and seductive sort that grabs lbo-ers attention...
In any event, good portion of US left probably hadn't paid much attention to popularly received 1978 uprising that brought People's Democratic Party (PDPA) to power, hadn't taken note of treaty of friendship and assistance that Taraki's government signed with USSR creating basis for Soviet military assistance, and wasn't familiar with PDPA program of land reform, literacy campaigns, equality for women (abolishing forced marriages & bride sales while granting women right to work), and cultural rights for national minorities.
Amin faction attempted 'rapid reform' of countryside generated peasant opposition that was welcomed by reactionary elements and previous ruling interests: wealthy/powerful feudal lords, tribal chieftains, traditional clergy. Internal PDPA split destabilized already difficult post-revolutionary situation and made task of moving country from feudalism to modernity even more daunting.
Soviet leadership rejected early request for military assistance by Taraki government clearly committed to progressive social change. Decision to pursue military action was only made following failed attempt to negotiate political settlement that would have created broader coalition gov't and slowed pace of reform (both Amin and fundamentalists rejected proposal) and Taraki's murder at hands of Amin who had apparently sought US help, meeting a number of times with then-US ambassador Dubs.
While one didn't have to 'hail the red army', one should have been able to see that Carter would use Afganistan to justify policies already being prepared - the way Truman used Greece & Korea and the way Johnson used Gulf of Tonkin. But left anti-communism in US was pervasive enough to prevent many from accepting that Soviets were trying to bring order to chaotic situation in neighboring country *and* trying to prevent setback for forces of socialism (even if it was socialism made under circumstances not chosen but under circumstances directly encountered, given, and transmitted from the past). Michael Hoover