You're confusing ethic with effort. I agree the wage impacts the effort, but you can counter that if the leadership is strong.
And 17 internals in a year is insane. That speaks more to poor hiring practice than anything else. Thats out of your control.
17 isn't that insane, even if they were all AP cash/merch cases. If you think that in a store of 150-200 TMs or more there aren't at least 10% of people stealing when 30% of the public has a criminal record and about 9% have felonies, then I don't know what to tell you.
I do think a positive store and company culture leads to less theft, but there's always going to be some, so having 15+ internals a year in a place that employs mostly lower income people isn't a surprise.