They are probably gauging how prevalent and what level of quality fan material will be. If (for example) Several site publish high quality pdf's that extend 4E far beyond the strictures of the GSL, WotC will have to step in and shut them down. This would be especially true for fan sites of popular (relatively speaking) campaign settings like Dragonlance or Dark Sun. If, however, fan sites release small low quality additions that simply fill limited niches, WotC can sit back and not come down on anyone too hard. If they take a hard line stance immediately, they run the risk of losing the good will of the fans and there by losing 4E sales. It's all a matter a perception.
Again, this comes down to copyright/trademark law. Legally, pretty much every setting WOTC/TSR has published is heavily loaded with trademarks, and there's plenty of established legal precedent for that. It's VERY unlikely WOTC would ever issue a formal policy which said "You can use our old trademarks freely, as long as don't sell it" or the like. (They might do this for specific sites via a privately negotiated contract, but it will never be broad policy.) If the issue is "how much can we look the other way", then that's an issue for internal debate but it will NEVER show up in a formal policy; any 'fan site policy' will be written so as to give them the broadest possible net the law permits, just in case.
As for 'waiting to see what fan sites produce', since many people are waiting for the policy, isn't this counter-productive? Besides, WOTC doesn't have the manpower to police GSL submissions -- do you think they'll be scouring fan sites to see if "The Net Guide To Warlocks" is up to their standards?
Really, again, this is an issue where the OGL worked perfectly -- it applied to non-commercial ventures as well as commercial ones -- and the GSL is nigh-useless. WOTC has thus far been mute on precisely what they hope the GSL to accomplish, so it's hard to judge how much of a success the GSL will be in the commercial area. Dancey's goals for the OGL might have been hubristic, but they were at least clear. The goals for the GSL? Not so much. Ditto the fan site policy.
It's hard not to fear that the fan site policy will be designed to limit/reduce competition for Gleemax, which wants to be the "MySpace for geeks". The current poor performance of Wizard's boards and the slow rollout for DDI isn't exactly making people look to Gleemax as the hosting solution of choice; the fact it will be ever under the watchful eye of Hasbro also seems like a threat to creativity and freedom of expression. I will assume they will offer all sorts of incentives for people to use their hosting for fan sites, but it's hard to guess what kind of goodies they can offer which will overcome the negatives and poor initial launch. First impressions last a long time on the net.
I suppose we'll see when we see.
