This exactly the issue I tried to point out earlier. There is a big difference between CDL qualified dock and city drivers running road VOLUNTARILY as casuals on their days off and on the other hand those same workers being REQUIRED to run road during their normal work days/hours. IMHO if the job uses road casuals then dock/city should be able to do that work voluntarily on their days off as long as they protect their bids.
As a PS, I ran as a road casual at variety of Teamster jobs in the past - Eastern Freightways, ST. Johnsbury, Pilot, Conway Eastern Express (former Penn Yan, Teamster job) - when I was off/slow at my seniority job.