Someone could intercept a floppy and read/write its data, but putting malware on it and getting the receiver to execute it unnoticed is going to be much harder than with a USB drive. Putting a floppy into a drive literally does not cause any code to execute - it's only when commanded to, i.e. when the drive is accessed, that things start happening. There's no AutoPlay for floppies. Contrast this with the whole USB enumeration process.
I also wouldn't trust a floppy controller's firmware any more than I would a CD or USB controller's firmware.
Opening an envelope is easy, doing so without being detected day in day out is hard.
Actually there was a joke going around here in Germany when the NSA scandal broke, talking about how jealous all the ex-Stasi guys must be that the NSA had beaten them at their own surveillance game...
Our customer had edited a doc on her husbands pc and got infected - the pre boot checks caught it.
http://www.amazon.com/Sabrent-External-Floppy-Drive-FL-UDRV/...