You and I might agree they shouldn't be concerned about it, but the government is not in a position to set internal company policies. If a company has a policy that if you have a drug conviction then you're not getting hired, if they get that record, you're not getting hired.
There are also other factors at play. Some of this has to do with bonding or insurance issues. Their insurance provider might say that they can't have somebody working on a job site if they have a conviction record. There's nothing—at least that I can see—that Parliament could do about that. The simple thing to do is to remove that record.
While we may all have a very enlightened view, saying that it was in the past and it's now legal, so come work for me—I'd probably feel that way about a prospective employee—you can't legislate that, other than removing the record of conviction.