According to the fine print:
"You cannot deploy the promotional Crystal Enterprise 9 Professional five-named-user licenses on any existing Crystal Enterprise 9 deployment or stack promotional licenses to increase the number of named licenses. This promotional offer is restricted to a single standalone deployment of five-named-user licenses."
You should be able to add additional licenses that you purchase after the you've received and registered the free software. All in all, this is a pretty good deal. You get a fully functional version of CE instead of the worthless standard edition. What you don't get for free (at least from what I'ved read) is Crystal Care Technical Support or Maintenence (the ability to upgrade to the next version for 'free').
In the past Crystal has given away Seagate Info and Crystal Enterprise 8 Standard. I don't think has much to do with Microsoft. Its more likely due to the fact that they're pushing Crystal Reports 9 instead of Crystal 8.5 and 8, which included Crystal Enterprise 8 Standard (not compatible with CR 9).
As for getting 'hooked', I've seen many posts on these forums from individuals who've used their free-version of Enterprise in production environments and then freak out over the cost of upgrades, additional licenses, tech support and maintenance. Even though this is a fully functional software, it is still intended to be used for evaluation purposes only.
