Simple test - which kinds of incentives one uses for purpose of having anything done:
- market incentives (do it and you'd be rewarded)
- force (either you do it or you'd be deep in trouble)
[Sure, there could be some social /religious / ideological pressure, but at the end of the day you just do it because you'd gain / not lose your social capital]
As one see there is a big problem from communists perspective. Sure, one may reward top workers and Party members with ex. access to deficit goods. However, if that is being used on bigger scale, then you start getting for practical purposes high level of social inequality. (Sure, everyone can buy a car, just a Party member would skip 10 year queue and actually get it, instead of having it just vaguely promised like the rest of society). Some level of inequality and semi-market approach could be applied without being accused of failing orthodoxy, but still force is necessary.
How can one keep highly skilled and high working labour force, when if they had a freedom they would escape to capitalist countries and earn there more? East Germans at first thought that they can allow an open border, as they would get a safety valve (like loosing potential trouble makers) and not a severe brain drain.
Similarly, even kept inside, people may come up with their own business ideas. Either one squash them (or at least cap their growth potential through draconian regulation) or soon would face nimble entrepreneurs outcompeting government institutions.
So yes, in order to have communists society you need to force people to do multiple things that they don't like, including forcing them to stay. Authoritarianism is not a degeneration, but logical choice when you have to force people to behave according to ideology expectations and have limited ability to buy them off.
EDITs:
For my defence of having allegedly excessively negative approach towards communism, I'd like to mention that's presumably a result of having a misfortune of actually having to live under communism.
Motivation - a capitalist country could recruit soldiers by offering them extra money or by using their patriotism. Nevertheless, when those motivators are exhausted, and there is no one left who would willingly enjoy trench warfare, then it has to resort to conscription. It's not that communism does it, just every system when faces running out of options, has to resort to brute force. The problem is, that by definition any more egalitarian system would sooner exhaust all economic motivations and would have to rely on force more often.