It is weird that the ones that do won't be allowed to advertise that fact (assuming this report is true).
Not really if a couple of other factors are true.
1. Those retailers are likely going to get a relatively small supply in the first couple of days. Perhaps some will only get demo units. Or a small amount, so that shortages are almost instantaneous. In that context, advertising only makes the situation worse.
Likewise if retailers know they will get some later in the month, don't really want them advertising about having iPads available with caveat that will be available later in the month. This first weekend folks will be bouncing off the walls just to find one. No sense in adding fuel to the fire. If the iPad is somewhat hard to find later on, a later announcement will still draw folks to their door/website.
2. This practically assures that any mob of folks who haven't preordered will mostly line up outside of Apple stores. That will be great for the local news stations to take pictures of and deliver free advertising for Apple. In locations where Apple stores are not prevalent the local news crew will go to a Best Buy (or local vendor that is a store gap filler) . As long as the targets for the mobs to go on the first day is limited, they will likely go and gather in numbers that makes for voyeuristic interesting video.
3. Apple makes more profit when they sell iPads themselves. Stores make some money selling iPads. They aren't selling them "at cost" so they can make nothing. So when Apple Store (online/physical) Apple gets to keep that profit, also. This tactic makes sure that Apple maximizes their profit on April 3rd. Other retailers just have to accept getting screwed one weekend so that can get profits later on.
4. If there are any launch glitches Apple only has a relatively small number of vendors to coordinate will. ( Anyone remember the 3G , mobile me , OS 2 upgrade debacle ? ). Keeping things simpler at first typically works better. They can add additional logistics headaches after this weekend.
The number of users who want one will not be exhausted on the first weekend; it is a reasonable trade off. The vast majority of this year's, US sold iPads are going to be sold between the end of April and Christmas. The frenzy of this weekend is a temporary blimp if look in a long term perspective.
If you are a retailer who competes against Apple and its stores .... gotta to expect you are going to get screwed from time to time. When your are remora swimming with the shark ... have to stay out of being in front of its mouth least you get eaten too.
