I wrote a little answer about this poem on the Literature Stack Exchange a year and a bit ago: https://literature.stackexchange.com/questions/493/why-does-... (See also the answer there by ”Rand al'Thor” for useful historical background—referencing the Guardian article mentioned in another comment here—and the succinct answer by Mark Baker.)

There is an apparent contradiction in the poem (which is what the question there was about), which disappears on a closer reading.