Why? Because if you put in motion regulations which players must follow and pass tests for, and they indeed pass, then said player is clear of fault. It doesn't matter if he gains 50 lbs and hits 99 homers, if you can't fail him at a test, how can you persecute him?
This is the end of the agrument.
MLB fucked this whole situation up - in 98 they promoted this whole thing during the McGwire/Sosa run, so they're complete hypocrites if they try to change things for Bonds.
At the end of the day, Bonds' numbers will prove him out to be the one of the best ever if not, THE best ever (there will be legimate agruments that Ruth destroys everyone since he outperformed his league by more than anyone ever will, but that's a different argument). His 73 HR season may be, by far, the best season anyone has ever had.
Imagine this conversation 50 years from now:
"Why isn't Bonds in the Hall?"
"He cheated, he used steroids"
"Oh, he failed a drug test?"
"Well, no, he never failed a drug test. He did gain a lot of weight since he was a rookie, and these guys wrote a book based on leaked grand jury testimony, and he hit a lot of HRs late in his late 30s and early 40s, though."
It doesn't hold water. Yes, I think he used. But I think so many used during the era that you just can't punish the best player of that era. It's not fair.