The very basis of your question poses a huge problem. Who defines terrorism as indiscriminate violence by non-state actors? What "terrorism" means depends upon who you ask. There is no definitive definition - one study of 55 academic articles found 73 different definitions! [1]

This means that whilst, as Randall Law has argued, ‘terrorism is as old as human civilisation’ and that ‘core features have remained since the earliest times’, terrorism has largely been defined in the same way Justice Potter Stewart famously defined pornography - 'I know it when I see it'..[2] This has meant terrorism has often been defined emotively and has been approached as a pejorative term. This, Louise Richardson has claimed, has resulted in the only universally accepted definition being that ‘terrorism is something the bad guys do’.[3] This has produced huge problems - as highlighted by Stephen Nathanson who has written a highly interesting philosophical work on defining terrorism - as the idea that we should define “terrorism” by who does it leads to absurd results’ and a complete loss of objectivity.[4]

Returning to your question, how this has lead to widespread views of terrorism being indiscriminate violence by non-state actors depends on where you are from and who you talk to and giving a precise answer is impossible. For example many in the Middle East point to America/Israel as being the true terrorists etc etc.

There are, however, two main factors that can explain it in part both based off the assumption that you are based/living in a stable nation-state and from the clearly written English of your question that these are either the US or UK.

First, is the widespread domination of nation states and nationalism in the last century (Marxists to use your example are by definition not nationalists). This has meant terrorism has often been attributed to non-state actors, as these are usually the "bad guys" (IRA / Al Qaeda / Hezbollah / Tamil Tigers etc). The pejorative nature of terrorism as a term in turn means that similar actions committed by the US military for example will not be described as terrorism.

Second, is 9/11. 9/11 is a huge outlier in terms of casualties and the response to it marked a historic shift. For as George W. Bush recorded in his memoirs he viewed 9/11 as a declaration of war rather than a criminal act, which in turn lead to the "war on terror" which has touched every aspect of life in the West and has costed trillions of dollars. Again this has reinforced the non-state dynamic of terrorism as this war has almost exclusively been state vs non-state actors - US/UK/Allies vs Taliban etc.

TLDR. It's impossible to answer when terrorism became widely defined as indiscriminate non-state violence because terrorism is defined differently wherever you are. Assuming you are talking about the West then this is due to the domination of nation states and 9/11.

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[1] Leonard Weinberg, Ami Pedahzur & Sivan Hirsch-Hoefler, ‘The Challenges of Conceptualising Terrorism’, Terrorism and Political Violence, 16.4 (2004) 777-794 (p. 780).

[2]Randall D. Law, Terrorism: A History, 2nd edn (Cambridge: Polity Press, 2016), p. 1.

[3] Louise Richardson, What Terrorists Want: Understanding the Enemy, Containing the Threat (New York: Random House, 2006), p. xi.

[4] Stephen Nathanson, Terrorism and the Ethics of War (Cambridge: Cambridge University Press,2011), p. 13.