It is all just a choice of the Kickstarter Project Managers. I've seen projects go both ways myself. Some have even done physical delivery's of some bonus or add-on content before the main delivery. There are logical reasons for Monolith to do it either way. In my mind it would just make more sense to give out those keys for an AoC pet before the physical delivery mostly because it is something that could be used right away, for a product other than what the kickstarter was about.