IF you modified it and used it in such a way as to intentionally interfere with otherwise licensed (or license-free) spectrum, then hell yes, you could be.

Your microwave oven, as designed to be used, is certified for it's intended use. If it was leaky by accident, and interfering with stuff though.... you'd have to not use it - and continuing to use it after a warning would put us back to the first part of this discussion. It may not cause harmful interference.

Microwaves are fcc approved and designed to minimize interference.