I am not a fan of Rupp, but I have no problem with what he did - seems entirely reasonable to me, as does his/Salazar's explanations. In contrast, the critics here have offered up some pretty lame critiques.
To those saying Rupp's 10k would benefit from marathon training ... I think we can assume that Rupp has been doing essentially marathon training. The issue is with actually racing a hard marathon and the recovery that would require. By doing the training but not the race, you get most of the benefit without the need for a long recovery.
To those saying it was just a ploy to mess with other competitors ... how would it mess with people? I don't see any of the OT qualifiers altering their training whatsoever based on an announcement that Rupp had entered the race.
To those saying it was a PR move ... I don't recall Rupp or Nike or anyone promoting his running. What PR there was came just off people noticing his name on the declared list. If you wanted to get a big PR bump, it would have been easy enough for Rupp/Salazar/Nike to say something about it, even just a tweet like "Thinking about running the OT marathon."
Finally, for those whining about this ... have you ever waffled on running a race and not decided for sure until a week or so beforehand? Most runners I know have done so.
Again, I don't particularly like Rupp, but I think the critics here are, in this case, even lamer.