And arguably, favoring high-UA commenters makes sense:

• let's for example take @blocktrades as a commenter on this post. @blocktrades is a top witness, but also a high-SP account and runs a crypto OTC (?) conversion trade service that doesn't require registration to be able to use it;
• when I joined steem(it) at that time all exchanges were closed to newcomers and I had no account there, so my only way to buy BTC was via @blocktrades . So for me, @blocktrades "saved me";
• probably a lot of other people feel the same way!
• which justifies the high UA-rank of @blocktrades , because UA is an influence metric. If it weren't for their service, I would have probably left or at least not (so) active. But because of @blocktrades I stayed. That makes @blocktrades very influential, for me at least, and probably to a lot of people.
• very influential people get bombarded with interactions, opportunities, they need to be very selective;
• so if and when a very influential person (having a high-UA rank) feels the need to comment, then the post is arguably important and interesting enough to engage with.