Airclues
A piece of freight is to be moved by Polar, it has a Polar airbill and Polar will gain commercially from its transport - only they can't get the job done because their crews are on strike. Polar can try to ship this freight on a different carrier- say FedEx, but it's struck goods and stays that way so long as its carriage results in remuneration for Polar. A unionised carrier will not take this freight so long as it has a Polar airbill.
The customer of course is free to drop Polar and take the freight directly to FedEx for shipment. Polar has now been deprived of commercial gain so the freight is no longer classified as struck goods and will ship on the FedEx system.
The customer gets the freight moved, FedEx gets paid for it and Polar loses all commercial gain from the transaction.
The picket line issue is a little more ambiguous. Polar/Atlas have an incestuous relationship due to having the same holding company. Should the freight fly on Atlas the same coffers are ultimately credited as would be the case if it had flown on Polar as originally scheduled, hence the enthusiasm for Polar's striking aircrew to target Atlas aircrew with picket line action. Both being represented by the same union, ALPA, in theory results in recognition of the picket line and legal protection for personnel not crossing it. Recognition of the picket line by other ALPA carriers can result in various actions to express support, never enough though to please those setting the line up in the first place.
That is the theory. Lawyers and judges get involved along with parochial politics, personal ambition and taking advantage of opportunity. It is a dirty confused business and there will be many unhappy campers bearing grudges over a myriad of issues for years to come.