"The calculation that underlies the Casimir experiment, a calculation based on the formula predicting infinite vacuum energy, shows the zero-point energy of a system consisting of a vacuum between two plates will decrease at a finite rate as the two plates are drawn together. The vacuum energies are predicted to be infinite, but the changes are predicted to be finite. Casimir combined the projected rate of change in zero-point energy with the principle of conservation of energy to predict a force on the plates. The predicted force, which is very small and was experimentally measured to be within 5% of its predicted value, is finite.[34] Even though the zero-point energy is theoretically infinite, there is no evidence to suggest that infinite amounts of zero-point energy are available for use, that zero-point energy can be withdrawn for free, or that zero-point energy can be used in violation of conservation of energy.[16]" 

they're still subject to conservation of energy, but they can draw from the theoretically infinite form of zero point vacuum energy. by no means can they draw the entire output (the output is finite as said in the quote) but it's a safe and high tech form of power generation.