Of the relievers that were cut, Viola had the most impressive spring training. Then, I'm sure Baker heard all about Viola's huge tear in AAA in which he went all those consecutive appearances without allowing a run. For the most part, Viola has been dominant, and when you combine that with him being left-handed, it makes sense to bring him up.
If you want to make the argument that Fisher would be a better call-up, I wouldn't disagree with you, but I'm refreshed to see that the Reds aren't letting options be the determining factor in who is on the team--instead, they're trying to put together the best team possible. If the current trend continues, we could see the Reds cut bait with Mike Lincoln and bring up Fisher or Roenicke around the All-Star break. It's also inevitable that injuries for another call-up anyway.