Why do the fairies attack Seth in the novel, Fablehaven?

The fairies attack Seth in retaliation for changing the fairy into an imp. Grandpa is amazed that Seth managed to capture one and realizes that he did not specifically tell Seth not to because he never imagined he would succeed. After Muriel changes Seth back to his human form, Grandpa warns him that he has to use the experience as a lesson and that he must respect all the creatures on the Preserve.