"Telling or forcefully suggesting how yr employees should vote is the exact way to create problems among your workers and reduce productivity, " Cuban said.
Cuban also tweeted that he is an independent and gives "zero credence to political slogans." Still, the tweet is likely to add to the controversy surrounding Siegel's email.
As reported by CNBC yesterday, David Siegel – the CEO of the Westgate Resorts time-share company – said that if Obama wins re-election and raises Siegel's taxes, he will likely have no choice but to lay off workers and downsize his company — or even shut it down. (Read more: CEO to Workers: You'll Likely Be Fired If Obama Wins )
"If any new taxes are levied on me, or my company, as our current President plans, I will have no choice but to reduce the size of this company, " he wrote. "Rather than grow this company I will be forced to cut back. This means fewer jobs, less benefits and certainly less opportunity for everyone."
Siegel added that he might even close the company.
"I can no longer support a system that penalizes the productive and gives to the unproductive. My motivation to work and to provide jobs will be destroyed, and with it, so will your opportunities. If that happens, you can find me in the Caribbean sitting on the beach, under a palm tree, retired, and with no employees to worry about." (Read more: UK Rich Will Pay 'Fair Share' )
On CNBC yesterday, Siegel said he wasn't threatening employees, just "trying to educate them" before the November election.
-By CNBC's Robert Frank
Follow Robert Frank on Twitter: @robtfrank