I agree with jmilloy, actually; regardless of whether or not that usage is correct, given that poems frequently flout grammar "rules" I don't think using a poem as evidence of a particular rule is a strong argument.
OP said "I don't think anyone would ever use loose in that manner without preceding it with 'let'". I provided a counterexample, so the OP's statement is refuted. Now, granted anyone could write a nonsense sentence to refute any rule of grammar. But this is a very important poem by a prominent poet.
I think that my comment came across as an attack. Rather, I just find the ways in which poems can interact with and affect existing, seemingly fixed aspects of language are beautiful and interesting! And in particular, it means that poetry has a unique place in discussions about both proper grammar and common usage.
You make a good point that you were furthering discussion about common usage and not necessarily grammar.
