People shouldn't have to, but the reality is that it's probably the only thing people can do to be listened.
You can't simply vote with your wallet, because they simply don't know why a given sale ( and how many of them ) wasn't made. Lack of interest, price, the DRM how would they know.
And well it's also has more weight because not only they now might be missing sales ( even more so if you refound after leaving that review ) but now they will fear the additional sales they will be missing because of those review. So not only do you know you are heard, you also know you are making a greater impact than just not giving your own money.
As other people here stated, it also works well with how steam review works, which diminish/avoid long term negative impact if they end up removing the DRM :
- you can change your review
- there is a separation between recent and overall review