Why not? Why not sign multimillion dollar contracts when they are made available? In reality Nike, Coke and a dozen of other sponsor freak because of the parent who will group, protest and make a huge deal out of ^^^^ like this. Remember the infamous backdoor Kobe situation in Colorado which cost him millions? His sponsor sure didn't. He still isn't at this status before that even though the money is coming back.
The truth is that many parents allow their children to look up to athletes because its easier than having the looking glass on themselves or many kids simply look up to an "icon" in their chosen interests like Basketball players looking up to Michael Jordan or Football players looking up to Donovan McNabb.
What happens is when these people are shown as humans people freak because they honestly think that their kids will follow their paths exactly, mainly because some have made that misstep and it gets blamed on the star instead of the parent who didn't keep a better eye or from whomever was in charge of getting the message that there are differences between your life and theirs, so its a whole "what about the children" issue.
Michael Vick was an excellent athlete who worked hard and brought himself up from a bad situation and became a star. He also made a dumb choice by fighting dogs. Does that mean his achievements should be overshadowed? No. Does that mean that people shouldn't look to him as an inspiration? No. What does Gatorade freak and drop him? Simple, because PETA and the other dog lovers go ape^^^^ about them supporting a person who would do that and they play safe politics instead of risking wide spread protests and boycotts.
Them getting dropped and if anyone drops Michael Phelps it'll be simple business politics nothing more while people like to believe we are very much in an enlightened society that can separate life from achievements the reality is that the nation as a whole is still very conservative and show it with their dollar.