"On the earth shall you spill it as water": Just as it is permitted to derive benefit from water, it is permitted to derive benefit from blood — Just as water renders seed susceptible (of incurring tumah), so, blood. Just as water need not be covered, so blood (that of a disqualified offering which has been slaughtered) need not be covered.
(Devarim 12:17) "You shall not be able to eat in your gates": R. Yehoshua b. Karchah says (in respect to "able"): I am able, but not permitted. A cognate instance is (Joshua 15:63) "But the Yevussi, the inhabitants of Jerusalem, they could not drive out": They could but they were not permitted to do so.