Retail expects a level of costumer and service involvement not found in most markets. You're not factoring in the lack of benefits and lopsided work hours - front end employees at WalMart are especially strained through hours and underemployment. When I was younger I worked for Kroger (unionized) as a cashier, and I found the job more reprehensible than my first job as a groomer/pet carer in PetSmart (try washing and clipping two bipolar dogs, and having to care for the corpse of dead dogs). We had benefits, but the average pay for a cashier was $6.50. I can tell you from my experience in retail that cashiers do not "just" pass food over a scanner, and this attidue really infuriates me - it's part of the reason I'll gladly defend retail workers whenever a nasty customer comes around. They oftentimes have the responsibilities of a janitor, bagger, and stocker. There are speeds you must go (which is really inconvenient, because customers always have that one thing they forgot). You deal with incompetent higher ups who refuse to accept your excuses for not selling their credit card in less than 30 seconds. New cashiers are expected to learn at least a hundred PLUs (4011 makes for a good joke amongst cashiers. "Is that a 4011 in your pocket, or are you just happy to see me?). Sometimes late at night you will be the only front-end service person around and there will be a line that literally creates a fire hazard. So please don't try to pass off the excuse that they're overemployed. Retail is one of the most stressful experiences anyone could have (next to food service - and don't get me started there). Coupons are absolutely atrocious. You learn a lot about people from their use of coupons. Some will give you them upfront and remind you about their existence every three seconds. Others will throw them onto the conveyor belt, not realizing they can get caught beneath that machina. The worst can be found in people who wait to the last moment. If they forget before the transaction is complete, they'll yell for you to get management (as if that does anything - management oftentimes tells you to not remind customers of coupons). The fact people work 40 hours a work for the biggest American corporation only to get $20,000 - which is scarily close to the country's average - is despicable.. If wages had kept up with growth, a minimum wage job in America shouldn't be less than $15/hour. Just look at the growth rate for executive salaries and wealth.
$20,000 a year in most places is pauper change. Bills from insurance, rent, and health care would easily wipe over half that out, and food/car expenses another couple thousand.
A better solution than building up unions would be ending all the assistance corporations get from the state. The same assistance they've been getting since feudalism became obsolete: no personhood protection, no subsidization, no special tax loopholes, no incorporation process - period. Corporations are the most destructive form of business, and in a natural market cooperatives would beat them into the ground.