It should perhaps be explicitly stated: it's a private counterstamp. The Mint didn't put it there, the people who made those Lincoln-Kennedy souvenirs put it there.
While some collectors like to collect privately counterstamped coins, others see them simply as damaged coins. The people who do like to collect them, usually don't want to pay too much for them, so it adds very very little in terms of value.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis