Eric Greenwell wrote:
What regulation makes it illegal for a glider to use the TT21 between
15,000 and 18,000'?

Of course it's not illegal to turn on a class 2 transponder above 15,000
feet. But with a class 2 transponder, you can't tell ATC above 15,000
feet that you're transponder equipped, because legally, you're not.
Where I fly, I pretty routinely enter protected airspace at that
altitude, and I would hardly get a clearance without a transponder. This
has been the main reason for me to buy a transponder, so buying class 2
transponder would have been pretty pointless.

I don't care whether "they" would notice or not that I carry only a
class 2 transponder. The price gap of a mere $200 is far too small to
even consider to cheat.

Of course all depends on where you fly and to whom you want to sell your
glider later. So all I've wanted to say is that when you consider to buy
a class 2 transponder, be aware of the limitation.