Because he likes them too. They're all his friends, including Zoro. He was ready to die for Zoro in TB but we're gonna pretend that a gag moment in which he put razor blades in his food has more meaning in regard to character's relationships in a fictional story.
Do you have brothers ? Because that's exactly how it works. We fight them, hate them, show no affection out of pride. No, we don't pu razor blades in their food, because it would kill them. But in OP universe, feeding razor blades to someone like Zoro is not even remotely dangerous.