A shortish poem like this seems like a nice match. Ethernet has a minimum packet size, which is around the length of a line. The whole poem is short enough that you can remember it, but long enough that seeing a duplicate line would give you a good idea of having a network loop (possibly with delay) or having a second sender that started at an offset (although, maybe there's some ID information in the packet before the text begins).

This poem seems cute and memorable, and unlikely to offend any censors, and may have been available under a permissive license?

Could it be for trademark / copyright protection? There was an anti-spam mechanism in the past that worked this way. If you abused the system, they could sue you for millions. No idea if that is still around.

That's the other (and arguably more interesting) part of the puzzle! :)