Well managed unions maintain a strike fund so that their members can survive the economic hardship of a strike.
After googling a little I think you are talking about Graduate Students United - their dues are only $5/year, which means they aren't even in the same league as the serious blue collar labor unions.
Graduate students are a whole 'nother basket of beans.
I don't think it is fair to blame the large number of unrepresented workers on the union itself. I'm sure the UFW wants to increase their membership past the paltry ~5,000 it has.
Recent grocery store workers voted to strike and the pay was going to be $100/week for picketing 15-20 hours a week. Great.
For example, if you are hired as a driver for a delivery company, you, the driver, are responsible for speeding or running stop signs and red lights, not your employer. You never here about a manager telling their employers to speed and retaliating against those that don't speed to make a delivery. On the surface, similar laws may appear to victimize the people at the bottom, but it instead does something very very important. It's places a firm barrier across which almost all workers will push back from since they are the ones that will be left holding the bag when law enforcement shows up and starts handing out fines.
So in the original article, they could create a law that fines the manager some multiple of the amount that every one of those laborers lost. Let's say it is 15 minutes work times 30 employees at $12/hour. That's $90 dollars in lost pay. Now make the fine 10x that value plus court costs and lawyer fees and now every single manager is going to think twice before willingly victimizing these temp workers, and will question their bosses when asked to do things like this.
Laws that make illegal these unethical actions with very real victims would empower workers to stand up for their rights better than anything else. No one would ever have to stick his or her neck out, they can just call the cops anonymously and tip them off to unethical business practices. The enforcement of violations would be handled right there on the spot without ever involving the legal department of the corporation responsible for the abuse.
It's not ideal, and certainly no UAW, but it's much better than unchecked abuses. You can use funds to buy appropriate equipment like gloves for workers and for their replacement when they fall apart.
We already have a nearly flawless way to increase the welfare of low-paid workers, and that is to redistribute wealth to them through things like the negative income tax. Having enough money to live is the best negotiating position, union or no union. (For example, I think this is why middle-class unions have been so much more effective than lower-class ones.)