Somehow I agree with your point. But the thing is as I searched along in Google, many people use either way in describing their articles. Of course, I conclude that those sites are from the native speakers ( webmasters ).
Not necessarily, I think.
When referring to more than one group or more one type of things, I would go with a plural kind followed by a plural noun. The distinction is not always very clear though, so kind might work fine in both singular and in plural.
That means using the former is not wrong ? Or grammatically it is wrong but people tend to use it that way ?