In this case, the company advertisers need a benefit that should be exclusive to their advertising tier which is far higher than yours and uses a different set of legal agreements and business regulations.
Actually, I used to purchase those exact same banner ads periodically when they were available, and the process was identical to the process for purchasing the advertising that I pay for here, right now. The only difference is that there's no longer a blank banner space up there that says "purchase a banner ad here" (or something like that) right next to those two that are always up there. There was no difference in any legal agreements involved and there are no business regulations that apply. (If there are, please just post a link to those regulations for my edification.) The payment recipient was also identical; I don't recall it off the top of my head, but I can look it up if you'd like.
The ONLY difference is that the company that runs this forum has made the decision not to make those opportunities available to anybody but those two companies. The only reason I'm bringing this up at all or commenting on it is because your earlier comment about this implies that companies that advertise through banner ads are, somehow, more trustworthy than anybody who advertises here in other ways, which is definitely not an accurate implication. If it's a matter of any kind of objective criteria about prospective advertisers, the forum could simply publicize them and then require prospective advertisers to meet those criteria for banner ads. The only difference seems to be the decision by the forum to reserve those banner ads to those two companies, exclusively. I've never seen any other companies up there, either. Any idea why that might be?