Decimals. Fund their faucet. To see their system or hardware wallet to work with a number that large. There are lots and lots of reasons. I don't often get a chance to talk to our users, but I do ask when I can and if I can be nosy.
I don't want to be absolute. There might be usage of large amounts. It just makes no sense to me. Whatever I can program to work for large amounts will work for small amounts as well, unless I'm running a testnet faucet.
We do have a warning on the main market page.
Okay, that's good.
It takes time and energy.
... to test them. If you don't want to test the coins, then simply don't mine them. If you mine them for the sole purpose of selling them, then that just goes against the functionality of testnet which depends on tBTC having no monetary value. If you finished your tests and mined tBTC, then just give them to a faucet or to someone who needs them.