However, Kellogg's is soliciting job applications from the public. Automating the submission of a single application is clearly legal, but submitting many with the intention of overloading digital systems is clearly illegal.

IMO, this case is legally ambiguous because they are using a digital system to overwhelm a human system.

The public is 'authorized' to submit one application, so it's hard to say submitting many applications is unauthorized, unless done with intent of crashing servers.