Because the $100 for the plugin itself is for the work that went into creating the plugin
vBulletin / Internet Brands charge you $170 for a Branding Free licence for your forum.
vBSEO charges you $117.
Having branding increases traffic to our site, driving sales and helps us to provide even more awesome products.
Even if 1 in every 10k visitors to your site is actually a forum owner themselves (either in the present or in the future) and would be interested in our products, that's still 1 lost sale.
If we then assume that your site gets 100k visitors, that's 10 lost sales.
10 lost vB Optimise sales equates to $1000, which when compared to a $100 Branding Free licence is actually a pretty bad business deal for us
Hopefully that makes it clearer why we put branding in our mods, and why we charge such a high fee to remove it
Fillip