I think it's not ethical and it's obvious that they do it to increase their seo rank and to get some additional visibility however, I don't have a problem with this and I think it's legal and up to them.
On the side note I don't like their mods mostly because of the low code quality. They are doing everything with only money in mind. They are UK based company that based their business on fulfilling the vbulletin mods niche by using cheap outsourced coders from third world countries.