Here is where the scam word comes in to play. I am not saying they are scamming anyone, but I believe others bring it up for this reason.
When it comes to paid advertising of yours on another website, there typically are two possible benefits of it.
1) Visitors are going to click on it to view your website
2) The link is going to be helpful in regards to increasing your link popularity
So potentially there are four different scenarios possible
a) link provides visitors, but does not help link popularity
b) link helps link popularity, but does not have visitors clicking on it
c) link provides visitors clicking on it, and helps link popularity
d) link does not provide visitors, nor help link popularity
In the case with exactseek, scenario D applies where your advertising spend will not help your link popularity or provide traffic. Not a scam necessarily, but when you are spending money and not receiving any value some very may well judge that is the case.