I fail to see how this move damages Renteria's chances of managing again. Renteria has one season in the books, and his moves are there for all to see. John Farrell got another chance after Toronto; if Reneteria is any good, he'll get another chance as well.
There's nothing to be upset about: Maddon exercised an option in his contract, which was his right. The Cubs offered him a job, and he took it. He was under no obligation to wait or to pursue other opportunities. I've seen far worse in the real world, where loyalty on the part of employees or employers is considered a negative attribute that should be avoided at all costs.
If there was tampering, that's a league matter. Tampering is hard to prove, however, as GM's such as Theo are usually smart enough to avoid leaving a paper trail that would come back to bite him later.