"To discourage package authors from publishing packages written in TypeScript"
I tried to use it with private packages but that doesn't work either, apparently node doesn't even read the "private" field.
"You publish TypeScript source, and JSR handles generating API docs, .d.ts files, and transpiling your code for cross-runtime compatibility."
Not allowing it for private modules doesn't make much sense to me, though. It either forces me to use a loader, or now figure out a JS build step which I have been more than happy to avoid up until now.
As a past and present module author, I don't feel myself unduly burdened by the need to maintain the tooling to support the work I publish. Or present-ish, anyway; last time around it was Bower and about Node 8, and the experience these days is worlds more comfortable.
I can see why others would feel differently, but again, I'm not really here to argue preferences. If you'd like to fill your afternoon instead with an enjoyable, on-theme read, try the UNIX-HATERS Handbook: https://web.mit.edu/~simsong/www/ugh.pdf