All they conclude is that perhaps Santorum is holding back from endorsing Romney because "he just needs time to grieve" (for his lost campaign) and that once that process is done the endorsement may come.
The other conclusion they come to, is that Santorum may be considering that to endorse Romney before the Pennsylvania primary, would in some fashion "betray' his supporters.
They make the supposition that Santorum is not holding out for the VP slot and present this Santorum campaign mailing to supporters which went out just before he stopped his campaign. Based on this statement, which is worse than Bush's "Voodoo economics" attack on Reagan, they may have a point:
"It truly frightens me to think what'll happen if Romney is the nominee. My friend, Republicans and conservatives will be crippled by a nominee who presents zero contrast with Barack Obama on the major issues of this election. Any attempt by Mitt Romney to attack President Obama's positions on healthcare, energy policy, social issues, or the economy will be easily neutralized."
There are a number of other possible reasons;
Given the rank and file revolt in Colorado/Washington/Minnesota where Ron Paul has been racking up delegates and which included a Paul/Gingrich/Santorum backers coalition, and with Delaware coming up there remains the possibility that Romney might not get to the required number of delegates before Tampa.
If that were to happen then Santorum would of course be the king maker with the second largest number of delegates. In return for putting Romney over the top on the first ballot (after the first all delegates would be free and anything could happen) Santorum could get his conservative policies guaranteed-or the vice-presidency for himself or a person of his choice.
If Santorum does not endorse Romney, and Romney loses, Santorum will not be sullied in conservative eyes by having been a Romney supporter, and will be in a very strong position to seek the 2016 nomination as the party will doubtless look for a real conservative next time.
In perhaps a major way Santorum's future will be decided by whether or not he endorses Romney. It is noticeable that Sarah Palin too has not endorsed Romney although she has pledged to work for the ABO nominee. That would not be held against her or Santorum in 2016 as much as an endorsement would as it would be seen a simply party loyalty.
Santorum has one possible impediment that Palin does not. He has a massive campaign debt and one possible temptation would be to have the debt cleared by Romney's forces in return for an endorsement.