Meaning, they can sign with any team they want to, without trade or agreement from their current teams.
Not saying it's possible, but IF, CP3 and DH12 say that they just want to play together in Houston, they can just sign contracts with Rockets.
Now the trade talk is more about sign and trade - if unrestricted free agent sign with current team, they can get paid more, and they can carry that contract to a new team.
Why would Lakers and Clippers do that if DH12 and CP3 are leaving no matter how? Because they can get something in return, be a combination of players or some picks. Sure, it's not going to be dollar for dollar, or even just 30 cents on the dollar, but it's still something rather than letting unristricted free agents walk away free.
Why would players demand a sign & trade? Duh, they get paid more.
Why would receiving team do this? If you want to lure those top ufa over, you have to satisfy them - pay them more to get a sign & trade.
Again, not saying you can trade Lin with CP3 or Asik with Howard, straight up in a free trading market. It's under the special unrestricted free agency circumstances.
Again and again, that Sam Smith piece is pure speculation noise, but the trade talk itself is not necessarily a diss on Lin. Any owner/GM would seriously consider or just pull the trigger, if Asik plus some picks and other pieces to make salary work can get you Howard.
BTW, Al Jefferson is a unrestricted free agent as well, that's why his name was throwen into the fantasy speculation.
Disclaimer: I could be wrong. Any CBA rules expert please correct me.