In other words, the default should be "the opinions in this post are mine and mine alone..." for anything not explicitly done in the company's name.
But even if it were clear that the employee's actions did not fall into the category, a company is rightly concerned about anything that would create even a colorable cause of action (in otherwords, anything someone could sue for without it being immediately thrown out).
Also, the company might be worried about the appearance in the public's eyes even when lawsuits are entirely off the table.
It would be fairly simple to construct a law that protects companies from liability for things their employees say, but what happens if an employee wants to say... give bad stock advice and pays an employee under the table to do so on his own time?