As my initial reaction is confirmed by other comments, I write it as an answer.
Do not take any knives in your carry-on luggage to the USA.
Most USA flights have security just before boarding, either between piers or at the gate. I am not sure about Frankfurt but I am almost sure it has the extra check as well.
While European rules allow small size knives, up to 6 cm of cutting edge at time of writing, the USA rules do not allow any knives on planes at all and you can not be sure they use the European rules for the flight.
Maybe not as good a deal (but it might not differ as much as you might expect,) buy them in a normal shop in the country and pack them in your checked luggage.
There is a lot of competition for knife sales and I do remember seeing some good deals when I was last in Switzerland. And in Switzerland they do engrave the knives, so you can buy something very personal there.