In short, by purchasing the products, some people are concerned about indirectly supporting animal testing. A purchase might benefit the companies that sponsored the testing in the past, or it may allow them (or another company) to get a free ride on animal testing that they or another company conducted or sponsored.
Personally, I believe in change, so if it seemed like the company had sincerely turned away from animal testing, I wouldn’t have any issue with supporting them despite past actions. Of course, it’s hard to know with companies what the underlying truth is. I generally buy products that do not contain animal products and are not tested on animals. I don’t sweat whether the ingredients were formerly tested on animals… but then again, I also try to buy “natural” products, so animal testing tends to be less of an issue to begin with. I hope that helps. :)