Even if you did change the rules the old rules should still apply to hornemans as he broke the old rules while they were in place.
~ @Achrow
The point is that the old rules were vague as to whether what Hornemans was doing is punishable, since it was not specifically stated. With this case arising, staff members have amended the rules in order to specifically identify what Hornemans was doing as punishable. However, since the situation occurred whilst the old, vague rules were in place, Hornemans has not broken said rules and thus cannot be punished under them.
You could argue that by injecting money into the economy he was causing detriment to the welfare of the economy, therefore committing "acts against the server", however that statement in itself is incredibly vague and could be interpreted any number of ways. I know this probably isn't the place, but I would like to suggest that specific boundaries are labeled as to what qualifies as an "act against the server".
Nevertheless, I thank Fuzzlr for his response to this, as it outlines the situation in full and allows us the users to make reasonable and calculated conclusions.