As has been mentioned in other threads WOTC themselves have mentioned that you can do this. Its a little naive to think that the law is the be and end all of everything.
This is absolutely not true. I haven't seen a single quote by WotC in any other threads of this sort.
The only thing close to this, which they said before DDI was even available, was one staff member at a convention saying that if a family of gamers lived together, they would try and make it so that one subscription should work just fine for all the household computers. Which is what we ended up seeing with the 5 installs/updates per month.
Since then, though, they have said repeatedly that those 5 installs are certainly not intended for a group of gamers to share. Now, what they've said has largely been along the lines of, "Look, we can't actually stop you from doing so if you choose. But that isn't what the updates are for. And while it may not be a big deal to you, if people keep trying to abuse the system, eventually the suits upstairs will take notice, and institute extreme measures - limited updates, heavy DRM, and other things that no one wants."
I don't think they are blaming anyone for being confused by this, but.. yeah, it is against the written rules, and if enough people persist in spreading subscriptions like this, eventually WotC will have to find a way to prevent that, which will probably end up causing trouble for all other subscribers as well.