But you are right. Threatening the life of a president—regardless of political affiliation—is seriously wrong and should be vigorously prosecuted.
Did you read the poem? It is not a threat. It is a poem. A great example of bad poetry, but certainly not a credible threat to anyone other than the reader.
If this is a crime, then 99% of the political thriller books in print where there are threats to the president contained in them would subject the authors to prosecution.
If this law were applied even handedly, then all the producers of "24" would be serving out their sentences with Logan.