It's real simple, and someone with either a) principles, or b) courage would have said that . . . unless she really doesn't believe it, which I think is the case.
Because she's supporting him in his primary campaign. This would be the time to emphasize points of agreement (border security) while downplaying differences (amnesty). Her answer to Greta does that exactly -- it focuses on the border security issue and side-steps any discussion of amnesty.
She did “IT ALL” comes down to securing the border.