It's not without reason, I don't like people abusing free software in the manner the grsecurity people do it (https://grsecurity.net/agree/agreement.php). They license code under the GPL (because they have to) but then they effectively tell their customers that exercising their right to redistribute code will result in a termination of contract (which is at best against the spirit of the GPL, and at worst illegal).
I disagree.
> Everything is explained very well on the agreement page.
Yes, it's very clear that they're attempting to subvert the GPL by coercing customers into not exercising their freedom to redistribute (section 3, first sentence). I can't find it now (all I could find is https://news.ycombinator.com/item?id=11808914) but there was a thread about this issue earlier this year.
Essentially, in some people's view grsecurity is acting in bad faith which means that they are violating the GPL (not the license text itself, but the copyright law surrounding license agreements).
Even you said they're not breaking GPL multiple times. What they're doing is allowed.