That's not the right question. The question is whether ex-Presidents ought to accept these bribes. Compare the post-Presidencies of Bill Clinton and Jimmy Carter, starkly different.
> Yet obviously these celebrities aren't being tacitly bribed. What is your explanation for that?
Celebrities aren't in positions of public service, so the whole concept of a "bribe" doesn't apply.
Celebrities are different because they make tons of money and are business propositions in and of themselves, meaning their star power could bring immense attraction to the event, which is part of what commencement speakers are for.
One could take this further and apply it to the entire concept of a revolving door; if an official can make a career out of lobbying and consulting after leaving public office, they'll be incentivized to prioritize future employers over the public interest.
Realistically, it's difficult to fully remove perverse incentives and still be able to attract talent (this applies to technical recruiting as well!). But there still need to be systems of accountability. Unfortunately, with two-party non-proportional representation, and a limited opportunity for voters to provide referendums on the corruption of party members separate from their policy viewpoints, there are many layers of insulation that prevent people from being held to account.
This is not necessarily true. The Clintons, Bushes, and Obamas are like one big happy family. Trump is the exception because he pissed off everyone, but even he used to be friendly with the Clintons.
Anyway this misses the point: in general, the system of buying political favors only works when political favors are repaid, and everyone operating in that system knows it.
To add to this, we should also be holding Presidents to a higher ethics standard than, say, Kim Kardashian.
https://www.thoughtco.com/former-presidents-speaking-fees-33... Former President Jimmy Carter "seldom accepts speaking fees," The Associated Press wrote in 2002, "and when he does he typically donates the proceeds to his charitable foundation."
> Clinton runs one of the most successful charitable orgs in the world.
So does Bill Gates. So what? Clinton also made more than $100 million for himself in speaking fees.