This doesn't sound as cut and dried that they leave, as some seem to be suggesting in the media.
Quite right.
I've followed this like everyone else, from the outset. This is how it all appears to me:
1. The Super League was set up in secret.
2. The '12' had agreed to be part of it (whether they have signed legally-binding contracts, I don't know).
3. Massive public and media outcry against it.
4. Rather large demonstration at Chelsea tonight.
5. Man City and Chelsea have APPLIED to leave the Super league (not definitely left it).
6. Woodward has resigned from Man Utd.
Now, if the '12' HAVE signed a legally-binding contract to take part in the Super League, then the Super league may simply say 'No, you are not leaving, we have a contract with you'. If the clubs then unilaterally leave, the Super League may take legal action to keep them to their word and protect their 'product'.
Once the issue in the above paragraph has been clarified, it may be then that the Premier League and UEFA take measures against the '6'. Hence their not coming out with any definitive action so far. And this may be the reason why the other four have remained silent so far, to see how the wind blows on the matter.
I'm not claiming any of the above is 100% correct, just my weighing up of the situation so far as it is known to the general public at this point in time.