It's actually really simple. If you're non-CHL player under 20 when you're drafted, your team has your rights for four years from the day you're drafted. I think where the confusion came from is most players this has been an issue with were players who started college hockey right after their draft, so their four years coincides with their Senior year of college. Example like Petersen is different because he started college a season after being drafted, not right away. Changes things and skews it a bit and is much less common.
As for the bolded, if you draft a 20 year old college kid he has been playing against elite competition at the college level for two years even if he didn't play Junior prior to that. And now he still has another two years of that competition before you have to sign him. And ideally, if you're drafting someone at 20 years old you are probably more confident you are going to be sign them than not because it should be a bit easier to project, in theory. They are older, further developed, you shouldn't need four years to decide that. That's how I look at that, anyway. You also don't see that happen all that often, anyhow.