How is it legal to add ads into an already sold product afterwards?

I assume that, unless they advertised it as being free of advertisements, that there is no conflict with them adding them into the product later. They can make even make an argument that those ads add value: wow, you get to learn about all these cool products that are relevant to you! But in general, I don't believe inserting ads into an existing product is against the law on its face. It's a shitty thing to do though.