A little historical context is important. RFK was considered a neutral hitters park back in it's first run, in the 1960's. RFK has not grown at all in the interim. Rather, other parks have shrunk to the point where it is now considered a pitchers park.
To expand on NeedaNewNick's point, if you can buy offense cheaply, you can invest the savings in better pitchers. This is one of the objectives of A's management, as described in "Moneyball".
