Everyone in media and politics knows how this stuff works. There is no point in threatening when everyone knows you can't back it up.
Trying to threaten would just make you look weak.
Huh? Cruz can't back up the allegation that Trump was behind the story either, so why would that stop him?
Besides, Cruz didn't deny the allegations. As someone wrote on another board, calling them "garbage" is lawyer-speak for "I did it, but I'm trying to look like I didn't, and don't want to get in trouble later for lying if more proof comes out."
Trying to threaten would just make you look weak.
Not if you know the story is false. Blaming another candidate for the story makes you look far weaker. ;)