I've done a bit of googling and if I'm reading this correct it was always forbidden (or as long as the US Code has been around) to have products containing "a non-nutritive object", but it was re-emphasized for Kinder Surprises specifically after they investigated some imported ones. This was in 1997, while the MagiCan was 1990.
Anyway, reading the text there, I'm fairly sure that this thing would also have been illegal under that code.