They not only pay the fee (without scamming the witness discount), they also increase the fungibility of coins. Calling it waste because they use a couple more bytes than Luke personally deems acceptable is ridiculous.
Discussion
Tx0 does not increase the fungibility of coins, it's a self spend transaction without other participants. No other coinjoin implementation creates this waste.
You have to split up the coins somehow before mixing and have some proof they paid the fee. How would you do it?
Mixing already splits up the coins, and you can pay the fee directly in the mix. Wasabi Wallet, BTCPay Server, Trezor, and JoinMarket all do this already.
I got burned by the original wasabi implimentation where the fee paid during the mix went to a fixed fee address that wasabi used. This was the reason I switched in the first place.
Then I'm happy you're about to switch back to Wasabi since Samourai uses a fixed fee address: https://web.archive.org/web/20231025112815/https://code.samourai.io/wallet/samourai-wallet-android/-/issues/462
Oh, that's concerning