Looks like they're making a corridor so that scabs can get in and warehouse supplies can get out. Cops put themselves in between the business and the strikers so that they can treat any conflict with the business as an attack on the "neutral" police.

Israeli settlers employ similar tactics in Gaza. You'll have civilians go into Palestinian homes and onto Palestinian worksites, set up makeshift structures, and declare the property there's. Then the IDF hover over their shoulders, ready to open fire on any Palestinian who confronts the settler.

In this case, the cops on the line aren't the ones you need to worry about. It's the cops waiting at the APC ready to charge in, crack heads, grab picketers, and drag them to jail if any of the first group cries foul that you have to worry about.