The cap is tighter for everyone around the league.
Look at how NE had to "extend" Brady for 3 more years and in the process guaranteed the entire contract in order to provide cap room. Alot of the media folk are trying to spin this as a Brady took-one-for-the-team move but in reality NE just made his original deal a $60M guaranteed one. The other side of that coin is that NE is prolonging the development of their next QB. By extending Brady three more years, Mallet will likely be traded away and they don't have another QB in the fold to learn yet. It's a real sticky situation trying to stay competitive but also continuing to develop players you draft.
It doesn't take long but look around the league how alot of teams are restructuring contracts to provide cap room to sign their own veterans or to prepare for FA. The Steelers have done it already with Antonio Brown's contract (he just signed in 2012), Lawrence Timmons contract, and are looking at doing it with Big Ben's too.
The Lions, Giants, Saints, and Panthers have already done the same with other players to get cap relief.
What Stephen is trying to tell you is that the Cowboys do not want to tie up money into top tier FAs because they will need that money when Murray, Lee, Carter, Smith, and other young core guys need to be resigned. If they go out and spend big in FA this year it will almost be a guarantee that they will put themselves in a bad situation to keep all of those young core players they want on the team.
"It's little bit like the description of pornography from years back. It's hard to define it. But you know what it is at the end of it. It's hard to define it. I think you know who is more physical. Often times you see that in the fourth quarter. A lot of time the team on the other side feels it. We pride ourselves on being a physical football team. It's important to us. It's an important time of the year to lay that foundation." - Coach Garrett