One's employer should have nothing to say about your political contributions. What if all large corporations began forbidding their employees to give to Democrats? What if they began requiring donations to Republicans?
Once you begin giving employers a say in how individuals exercise their political rights, you are on a very slippery slope. I don't want my employer anywhere near my 1st Amendment rights and political deliberations.
As long as he wasn't on the clock and didn't use company postage or a company computer to do it and otherwise obeyed all applicable laws, then Olbermann should have had the right to donate to anyone he wants to.