Or it could also be that despite some top Safeties being paid so much not long ago, as teams prioritize the value of positions (and no doubt each team has a slightly different priority), Safety has generally lost some of it's priority/value with a sufficient number of teams that the supply is more than demand. And therefore when allocating cap space, barring a superstar, a "good" FS is being allocated less than some other positions or even what FS demanded even a couple of years ago.
Joyner is not Earl Thomas. He is not a superstar. I think under Phillips while the FS is a valuable position, it is less than some others. So while Joyner is good to very good, the Rams apparently aren't willing to pay him top dollar and would rather allocate the cap money to another position or player.
I have a hard time with the idea that owners collectively decide to virtually order their GMs that they cannot pay FS's very much in order to avoid collusion charges over 2 or 3 players. That act in itself would be the greater collusion than the collusion that is being alleged.