The employer is paying the employee to provide a service according to rules that both parties agreed to when the employee was hired. Insisting that they show up on time isn't coercing the employee, since it's something they freely agreed to as a condition of their employment.
"Restaurant patrons couldn't threaten to leave a (only one) negative review if their service is poor."
The restaurant patron is paying to be served, so they wouldn't be getting something they're not already entitled to by threatening to leave a bad review.
"but it would be illegal for me to request money from my neighbor and promise to tell his wife about his affair if he declines."
The difference here is that you're trying to force your neighbor to give you something you're not entitled to (his money) by threatening to do something bad to him. It's different from the boss or restaurant patron, who are demanding services that they've paid for.
I'm generally not a fan of laws banning blackmail, when the threat is not itself illegal. I find it odd that it would be perfectly legal for me to tell my neighbor's spouse that he is having an affair, or to request money from my neighbor, but it would be illegal for me to request money from my neighbor and promise to tell his wife about his affair if he declines. I'm not generally a big fan of Walter Block's approach to libertarianism, but this is his argument, and I do agree with it: http://mises.org/journals/jls/15_2/15_2_2.pdf.