Except he isn't defying grammar. It's correct to use loose that way.

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.

It's just that being in a poem doesn't have anything to do with being correct or not, whether it is correct in this poem or not.

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.