Unapproved Plastic Elevator Tips Question

Alright, here is a good one for you. Stene in Montana manufactures and
sells very nice fiberglas tips for elevators, stabilizers, wings, and
rudders, mostly for Pipers and Cessnas. You can find these parts advertised
in TAP and on the internet. I have inspected, painted, and installed
several of these fairings and was impressed with their workmanship. They
are very well-made. The plastic parts replaced by these Stene parts are
probably from Cessna. They are probably Royalite. Besides being
ill-fitting and ugly, they are brittle and crack and break of easily,
especially at the rivet holes. After a few years they get brittle and break
away.

An aeroplastics company in Texas sells these fairings too, and they are
PMA'd. I do not know how much more costly they are than Stene's but
understand the extra expense is substantial.

Now why should you be stuck with PMA'd parts when Stene's, which are NOT
PMA'd are just as servicable, just as durable? And if Stene's are not
legal, then how does Stene get off selling them?

Yesterday I looked at a 172 with all new Stene tips on the empennage
surfaces. They were quite beautiful. And, by the way, I saw this
aircraft's sister ship flying with most of the tips broken off and gone back
in the summer, so you can't very well say that the absence of one or more of
these fairings, or one having cracked and broken, is a hazard, can you?

What am I missing here, Gene?