They ARE NOT illegal in Wisconsin. Someone is blowing smoke up your rear orifice.
Headlight Modulators do not "flash" they modulate.
It's up to you to educate your law enforcement if you're pulled over for them. The Federal government allows their use in all 50 states. This is a fully authorized motorcycle safety device; that's Federally allowed to use on all state, county, city, land local municipality public roads, and overrides any ordinances to the contrary.