Homer, you 'are exploring the logic here.'
That's your mistake, you are looking for 'logic', good luck with your hunt!
The logic is that type-in traffic (direct navigation) is perceived as more desirable. According to Google, type-in traffic has a higher conversion rate than (in many cases expiring) link traffic.
Opening up parking programs to domains which are actively promoted would create little value to advertisers. Predictably, too many would use email spam to generate clicks, or try to attract clicks through mostly misleading link spam.
If you want to promote your domain, develop it.