From a lawyer:
If you can stand two e-mails from Wyoming on this topic, union organizing is the actual reason Wal-Mart and others are eliminating solicitations. The National Labor Relations Board has repeatedly held that if an employer allows an undefined amount of charitable solicitation on the premises, the employer cannot prohibit unions from coming on the property to solicit employees to join the union or sign a card calling for a union election. To make matters worse, the rule has no bright line — no employer knows with any degree of comfort how much charitable solicitation is too much. If an employer unwittingly crosses the gray area, and then refuses to allow a union to come onto the property, the union can file an unfair labor practice charge alleging the employer discriminated against union organizing in violation of federal law. Wal-Mart, Target, and the rest are likely recognizing this risk.
If you would like to see the application of this rule taken to its absurd extreme, review the opinion letter of the NLRB General Counsel issued shortly after 9/11 as it relates to charitable solicitations arising from that event.