What Policy is Advanced by Employer Captive Audience Meetings?

Did you catch this story in the Globe about how Wal-Mart is forcing employees to listen to their views on which political party they should support?  Wal-Mart doesn’t like the idea that the Obama camp is sympathetic to the recent push to legalize a card-check certification model that would allow unions to be certified without a vote if they gather majority support in the form of membership cards.   This is the model we still use in several Canadian jurisdictions.  Wal-Mart, that guardian of employee interests, is holding captive audience meetings in the States intended to persuade employees that they not support the law, and presumably its supporters.
Personally, I find the idea of my employer trying to tell me how to think about matters unrelated to the specifics of how to do my job very spooky and patronizing.  But do you think the law should prevent employers from ordering employees to attend a meeting to listen to the employer proselytize its views on things?  I have stated my opinion on captive audience meetings at which employers try to persuade employees to reject collective bargaining very clearly.  I don’t think they shouldn’t be permitted, at least if unions are not permitted the same opportunity.  Employer’s have lots of other less invasive ways to express their views on unions (how about a letter?) without forcing employees to sit through prepared speeches when they should be working.  The British model makes sense:  it requires unions to be permitted into the workplace to address workers about the unionization decision to a degree a least equal to the extent the employer used meeting to address workers.   In Ontario (and most jurisdictions in Canada) and the U.S., the law permits employers to hold captive audience meetings without providing unions with any opportunity to talk to workers at the workplace.  
There is no question that our model encourages very unequal access to workers during the union organizing campaigns.  The question is what policy objective is advanced by this model?   And here’s another question:  Do you think a Canadian employer could hold a captive audience meeting of employees to persuade the employees to vote Conservative (or some other way)?  If not, what law would prevent this?