If I buy a cd, it's not stealing if my friend borrows it whenever he wants to listen to it.
Or am I missing something?
It would be a different story if I copied the cd for him.
Legally? Yeah, you're more or less right (not stealing, copyright infringement, but whatever -- illegal all the same).
Philosophically? Any time you share something with a friend, you're depriving someone somewhere of a potential sale. Which, with certain business models, is Serious Business. Heck, if you fast forward through a commercial, you're stealing TV. You don't buy your Kindle books, you
licence them. Etc.
WotC-wise? I'd be surprised if the designers themselves made everyone at their tables pay for a DDI account. They're probably well aware it happens. They probably wouldn't encourage it, but it'd be Bad News if they cracked down on it, so as long as it doesn't get to be a whole thing, they'll probably quietly allow it. I'm sure they'd be happier if everyone bought their own account. They might hope you take a whole, "If you like it, you'll want one of your own!" stance.
Mostly they won't encourage it actively because of the aforementioned philosophical problem, wherein normal social exchange becomes a threat to a business model. But that probably won't change what they do.
We might call this
corporate cognitive dissonance.