If I understand this right, in Casper the right chain is the one in which the least stake is destroyed. In one version, stakers are heavily penalized for going offline, so the chain you describe would destroy a fair amount of stake.

This is Vlad Zamfir's version; Vlad is the lead Casper esearcher, and takes a more long-term theoretical approach, while Vitalik is more focused on near-term practicality. Vitalik's version doesn't penalize dropping offline as heavily; I don't know whether there's some other mechanism to cover this, or he just thinks a lighter penalty is sufficient.

Thanks for the question! Until now I didn't realize the reason for the offline penalty.