A Walmart is a big room with some stuff in it. The average store is about 105,000 square feet, and has 225 employees. If you got the whole staff to stage a sit-down (unlikely, as there are always cranky holdouts in even the most successful organizing drive), each employee would have to blockade 500 square feet of shelf space--a little less if you account for registers, bathrooms, and so forth. There's no bottleneck, no place that's easy to block off. On a busy Saturday morning, any striking workers would be lost in a sea of customers.
To break a sit down, all Walmart would really need to do is hire a bunch of new associates (as I believe is allowed in the case of illegal sit-downs), and unlock the doors. Even if the workers blockaded the doors to keep scabs out, they'd quickly be in a position of having to attack customers to keep the store closed. At which point, any goodwill they'd accumuluated would evaporate, and the heavy hand of the law would wade in. Alternatively, they could start destroying the place, in which case, again, they would look bad and trigger legal action.And then, of course, there is the problem of proportionality. As Adrienne over at Adrienne's Corner points out, Wal-Mart certainly isn't the worst employer out there in the world of big-box stores. Of course, they've been a target for union action for quite some time -- amazing that the unions haven't figured out a way to go after Wal-Mart more effectively. A sign of declining union imagination, along with the decline in union power?