Advertising your company, brand or website is a must. Users understand this. However, it must be done in a fashion that doesn't associate your brand with negativity. By adding the advert on the plugin without an opt-out you are turning any potential buyers of future plugins off automatically. Sometimes you just have to give something away.
Let me explain:
I saw the flare on a site and liked it so much that I inspected and researched the code and the plugin directory to find it. I was even going to email the website owner if I couldn't find anything. I wanted it and therefore I went after what I wanted.
I had a very positive opinion of your company and even went so far as to decide to purchase the future plugins when they came out. I had formed my opinion on you based on your generosity of the fine looking and well functioning flare plugin. That is worth more than any link or ad. That is the very thing that marketing companies strive for: to make you want the product and do whatever it is they need to in order to acquire it. You had a golden ticket and a perfect concept of marketing your company and product. Then you got greedy. By adding the link and ad you limited your audience drastically because now no one is going to want to use the plugin. Sometimes you just have to give something away but the future rewards more than make up for the present losses. Anyone that saw such a nice plugin did the work they needed to do to find it. They didn't need the link. I hope you reconsider your marketing strategy and whomever it was that decided to add this plugin really, really needs to study marketing better.