96b meisa machmas melacha vs. regular oness

What is the basis for the chiluk between meisa machmas melacha and a regular oness where even the animal dropping dead from a heart attack would be mechayev you?
Ramban calls it peshiya of the mashil -- that the mashil did not verify that the animal is up to the task he is lending it for. Rashba says, since "לאו לאוקמה בכילתא שאילתא" and he realizes that the animal will have wear and tear from the work and yet he does not set up an evaluation of the animal's worth, it shows he is not makpid. And once he is not makpid on כחש, he is not makpid on death either מחמת מלאכה, because of the idea of מה לי קטלה כולה מה לי קטלה פלגא