If the issue is black and white, such as a conviction on record and a prohibition of hiring people with that conviction, it's pretty clear cut.
However, as Pat correctly noted, if someone has been 100% trouble free for 20 years since that conviction, should the conviction should still be held against someone? Also, some "convictions" may be improperly classified as more serious than they actually. Should a single conviction for public drunkenness or shoplifting 20 years ago deny someone from employment that has restrictions that you speak of?
In the real world, often decisions are made on issues that are NOT "black and white" and this is where secret data bases can be troubling. Suppose someone was arrested but aquitted of the charges. A potential employer might be turned off by that history despite there being no conviction and even if there is no official company policy or federal regulations.
More insidious is the fact there may be inaccurate stuff on your record. You say you find this argument amusing, but I don't think you'd be amused to find distortions or blatant errors about you in some privately maintained database that is hindernig your chance of employment or mortage or apartment. Errors happen, particulary if there are no controls. Also, there is the chance of malicious entries. After four years of college, it is only natural for a person to be involved in some sort of misunderstanding with a fellow student, roommate, faculty, etc.
These things do happen and people are hurt by them. It is very difficult to correct the record, especially when one doesn't even known where the "records" are maintained and by whom.
You say you'd "remember" any incidents, but obviously not gonna remember something that never happened. It's 20 years later and the database says you were arrested but acquitted. How do you prove it's wrong?
Older people have been fired from jobs because of some distant indiscretion -- properly disclosed -- as a result of newly past laws; laws that make no allowance for time passed. The authorities say "oh the law wasn't meant to apply to people like him", but the guy is still out of job and his life ruined.
An employer can, and will, do anything they damn well please. AFAIK, there is no law against discrimination because of family background (only ethnic discrimination is prohibited). An employer would just find an excuse anyway.
Very true, especially when the time since the conviction has been decades.