It's because people read more than one sentence and the very next one is:

"Failure to maintain active Thread Group membership while shipping Thread technology may result in legal action, including but not limited to licensing fees."

Which specifically mentions legal action in reaction to shipping only.

"Membership in Thread Group is necessary to ..." is just a statement of their wishes.

It's like "Sleeping is necessary for good health". It doesn't mean we'll sue you, if you don't sleep.