Are you saying you have to tell the developers not to test business logic, or that you have to fix the tests they write?
Test code should fail when an important underlying mechanism regresses. If it always fails on the next phase of the project because some hapless idiot needed to cover every line, the signal to noise ratio gets weaker and the purpose of the test code is lost.
Same goes for prototyping. I don't see much of a correlation between willingness to prototype and seniority, either.
Railing against code coverage is at this point just as dogma as insisting on it.
What I'm railing against is the idea that seniority is a prime indicator to the effective strategy parent comment insists on, which simply doesn't mirror my experiences. What I see is juniors picking up the habits of their superiors. They're learning this dogmatism from somewhere.
Willingness is one thing, but getting yelled at for learning and doing your best is another. I never treat my Juniors like that, and encourage a 75/25 working/improving split. Still, several frequently get anxiety about how long it's taking them to clear tickets.
The other thing Juniors don't get is prototyping. I regularly build something quickly to get the problem set into my head. If it's good enough, I'll leave it. If it's got issues, I'll throw it away and start fresh now that I know what I'm solving. Juniors don't work fast enough or effective enough to do things like that without putting timelines at risk. Plus, they frequently want to do some cargo cult methodology of the week like TDD.