This isn't safari doing it, its the website. You'll have to look for a way around it by going to a page other than just mlb.com.
Well, it partly is Safari's doing and part the website's, since all web-browsers publish their ID to the web server, who in turn base their response to the browser on this info (such as "Upgrade your Browser", or "Only IE is supported") or in this case, serve up the WAP version. On the Mac version of Safari, there is a hidden option that let's you publish any one of a variety of browser ID's. This was left in for testing, but works to trick some servers into letting Safari use an IE-only site (which generally works fine).
There should be an option in the Safari preferences to block WAP browser identification so sites like this or weather.com render completely. It is a pain and some sites, no matter what URL you enter, still revert to the watered-down WAP version for the iPhone.
It's ironic that sites that first offered WAP access for phones are now being cursed by iPhone users.