I don't think it is dumb. Why should the sender know what amount the recipient received? Why is that any of his business? What if the sender wants to keep half of it and forward the rest to someone else, prism-style?

Should the sender know that he sent $20 for the latest album but the merchant only got $10 of it, and the rest went to the artist and the producer, etc? I don't think so; it's none of the sender's business, the only thing he should know is that he sent $10 and the recipient gave him his product. He should not know how much the recipient received.

On lightning, thanks to multipath payments, the sender does not know how much the recipient received. On monero, he does. Monero is bad for recipient privacy in this respect and lightning does better.