A de facto assumption?

Solution 1:

A de facto assumption isn't bad.

An assumption shared by a people group or audience can be called a commonplace.

In ancient rhetoric, Aristotle developed the concepts of topoi, loci communes, and commonplaces. Within the invention process of rhetoric, the topoi (topics) are basic categories of relationships among ideas, each of which can serve as a template or heuristic for discovering things to say about a subject which a given audience will accept. "Topics of invention" means literally "places to find things," or loci communes.

Aristotle divided these into the "Common" and "Special" topics of invention, with the former being more general and the latter pertaining to each of the three branches of oratory (viz., deliberative, forensic, and epideictic).

Of the seven common topics, a commonplace (or de facto assumption, to use your phrase) would likely fit under the topic of "testimony"; specifically, proverbs and maxims. You can find some common assumptions in American culture in the following maxims/proverbs:

• Haste makes waste.

• A stitch in time saves nine.

• Honesty is the best policy.

• Confession is good for the soul.

• The Republican Party is for conservative, big-money types who want to hold on to what they've made through their own efforts and hard work.

• The Democratic Party is for liberals who believe that wealth is meant to be shared among the less fortunate and the downtrodden in society and that wealth is often made off the backs of the poor.