In my opinion, if a politician is being offered a fee to speak, there is no reason it should not be accepted. I do not feel a politician being compensated for his speech is any different from any other paid speaker. That said, I do not feel the politician should request to be paid to speak.
Politicians should be able to accept fees when they are offered for their speeches. Obviously the people extending the invitation want them to speak and are willing to pay for their appearance. Also, politicians are trying to raise money for their campaign and this is one way to do it. However, politicians should be careful about which groups they accept fees from and try to make sure they only speak to groups who don't hold opinions that contradict with their own. At least, they should if they want to be elected. I think money coming from PACs and groups who seek to influence their vote are a much bigger problem that simply accepting fees for speaking.
A politician is an elected public figure with a pay check that is provided through taxes. Since the American public votes a politician into office and then pays their salary, it is not ethical for a politician to charge or accept a speakers fee for speaking to the public which elected them.
I think the politicians who are accepting speaking fees are not at all in terms of our traditions in government. The salary of a politician should be regulated and they can't use the fact that they are public employees to gather more wages when they speak at conferences or parties.