On every airline I've flown, the card explicitly states "Do Not Remove This Card From the Airplane". In fact on Delta, that message is printed in block capitals, so it is safe to assume that the airline would prefer you not to take them. The in-flight magazine, however, is yours should you choose to take it.
As ChrisKen mentioned, it costs the airline a lot to constantly replace missing safety cards, but their presence and content are also mandated by the FAA. See:
https://www.faa.gov/regulations_policie ... ntID/22488 One could argue that few passengers actually read them, but that's irrelevant, because the FAA demands that a card be available for each passenger to access.
I would never remove a safety card from an airplane unless it was given to me by a member of the crew.