Nope. Programming. They have mission critical programs that allow them to act like slightly emotionally stunted but normal-ish people to fool people for extended periods of time. But when the program is no longer needed or the pathways to it on the chip are interrupted they revert to the cold, passive killers they really are.
When Cameron took damage to her chip and was in danger of being turned off she pretended to have emotions to fool John into freeing her, he didn't fall for it in the end but damn near.
They have no Human feeling but will mimic them into order to manipulate those around them as they see necessary. Just robots.
Cameron appears to have shown much more subtle clues that maybe she does have something resembling emotions though - not the unsubtle, and fake, emotions when she was trapped and still "bad", but little things. Like recently, reading the suicide-prevention pamphlet while looking thoughtfully at the ruined chip (of the self-terminated contortionist terminator) in her hand. Or when she was asking Sarah about religion, or asking John if she had a birthday, or when she was looking at the bar of Coltan she saved; or when staring at the primitive robots at the chess tournaments. She seems to be fascinated by her own origins, judging by this, and by getting some understanding about creation and life and death. Remember when she wrote the "goodbye note" (we don't know to who), after she had seen others do it for the girl at school?
And the satisfied smile on her face after she had been able to beat the nuclear plant guards at snooker (getting their money, and the info she needed about their ID cards)? She didn't need to fool anyone with that. Her performance as Allison, admittedly probably caused by some deep infiltration mode running by accident, was also extremely convincing, tears and all. And there is also the matter of her ballet performance, after the mission need for it was long over. Why did she do it, if she didn't get some satisfaction from it, which implies emotions?
Cameron appears to be such an advanced model, especially made to simulate or perhaps even experience human emotions (so she could work as an infiltrator), that I don't think it's entirely out of the question she does have something that approaches emotions - much like Data did (even before the emotion chip he seemed to show emotion at time, like in the episode with the "collector" who abducted him, for example).
Two other possible places where she may have showed some emotion: assuming the interrogation-Cameron in "Allison of Palmdale" is the Cameron we know, than the killing of Allison may have been inspired by emotions about the "lie" (it would mirror what she almost did to Jody, in the present, also seemingly on emotion). Purely rational, I don't the killing was needed - Allison lying was to be expected, and she had her uses still for study, to observe on and practice human behaviour, or simply to be "preserved' like the animals and people in the cages.
The other is when she reacted to Sarah's desperate question about cancer in "Automatic for the people": there she wondered if she was a timebomb to go off (this was a short while after she went bad), with some passion behind it.
