I know what you're referring to and what the problems are, but the thing is it's not so much exploitation as just a well designed marketting plan. It's exploitation of a weakness, where the game is so lacklustre without more characters which you have to purchase seperately, but this is the same as any other marketing scheme - they exploit your weakness of needing or wanting something, and they sell it for a price of their choosing. It's really no different to any other industry.
Although I find it gimmicky and overpriced I can see the appeal, but it's nothing new in the gaming industry of today where new characters, skins, vehicles, maps and weapons are sold digitally in video games - the only difference is that you have the physical figure and can choose to place it on a Portal of Power instead of download it from the internet and then choose characters from a menu.
£8.99 is a huge price for these figures especially when there's loads of them and don't ever see discounts unless they're pre-owned. For that much I can get 30 days of any subscription MMORPG of my choice, and I'd probably get more out of a day of that than a single Skylanders figure. £8.99 for a few minutes of 'extra content' is hardly worth it in my opinion.
So yes this could be regarded as exploitation, but it's just like any other brainwashing advertisement for children because they're so impressionable. The counter to this is just to not buy the game at all and explain why it's not good. Read some reviews and decide for yourself if these models are worth your money.