In theory, No, it's not "legal" ; on 'temporary' entry of non-US vehicle into US, you complete a US Customs form which promises to re-export the vehicle within 12 months. But there is no 'system' to track such vehicles or the entry paperwork after the vehicle is physically entered in the US. So, leaving without the bike, you've broken a promise.

In practice, Yes, it's "legal" for you to leave without the bike, because there is no US Customs paperwork or database cross-check/link between your personal passport/ visa, and the vehicle-entry paperwork. In other words, the vehicle entry is not posted/ stamped in your personal passport entry.

BUT...be careful trying to sell a non-US bike (that is legally entered by the owner on a 'temporary' basis) to a US buyer. If the US buyer then tries to register that non-US bike in a US State (rather than using it only for off-road use, not registered for street-legal use), most State motor vehicle agencies will cross-check the VIN against a separate national/federal database of "US-spec" bike VINs. If the VIN doesn't show up on that database, the State DMV will likely deny registration, unless/ until that bike has been 'conformed' to US-spec (a whole different & very costly system--don't go there). Many/ some US buyers may not even be aware of this procedure/ requirement. If the US buyer only intends to ever use the bike offroad and not ever register it as street-legal, no problem for either seller or buyer.

Or, as you suggested, a non-US bike could be 'parted out'.