Phrase that encapsulates flawed logic in making the excuse "everyone does it"?

Solution 1:

The bandwagon fallacy assumes something is true (or right, or good) because other people agree with it.

That's the last one of 15 Logical Fallacies You Should Know Before Getting Into a Debate.

And from Bandwagon Fallacy Examples on yourdictionary.com...

The bandwagon fallacy~ describes believing something is true or acceptable only because it is popular. The fallacy is also known as "jumping on the bandwagon" or argumentum ad populum ("appeal to the people").

In the common vernacular, people often mock this particular fallacy with variations on...

Eat shit, millions of flies can't be wrong!