"Gahaha!"

He's laughing because really, they should have expected Tonbokiri to think so practically and earnestly abou all of this. It's very on brand for Tonbokiri.

"Right, right, these watermelons cost us a fortune to get," He is, after all, a merchant sword so of course he would also be thinking of the financial reasons behind this, "Ya sure ya wanna use your spear, though?"

Some swords might think they're too good to cut produce.