Arrow USA's site shows street exhausts that are road approved...for Europe.
An exhaust has to be tested by DOT and approved on the bike during import procedures on new models
The only way ducati gets around it is they must test and approve their street Termi system on the bike during DOT certification
same way that no changes to lighting, reflectors, mirrors,fuel, exhaust, etc...are DOT legal...even models that are e-approved for street use in europe
(edit- as far as i know
)