It wasn't my suggestion, it's what the developer has stated.
They specifically said that the 3 second reduction was affected by haste, here's the direct quote:
"The base proc chance will be doubled and the cooldown reduction per proc will be 3.0 sec (affected by haste), roughly half the average expected value of current live Wild Call procs. Baseline, the amount of total Dire Beast casts and Dire Beast uptime should be roughly the same as on live" (parts bolded for emphasis)
So we know for sure that it is not a static 3 second reduction. There could be some ambiguity as to whether the 3 seconds is reduced or increased by haste, but this is clarified by the second part that I bolded. In order for the total number of Dire Frenzy casts to remain "roughly the same as live" it is necessarily the case that the 3 seconds is reduced by haste rather than increased, because this is how it currently works on live. Their specifically stated goal is to halve the effect of each proc, but double the proc chance. At 0% haste, we currently experience a 6 second cooldown reduction per proc on average, and this is halved to 3 seconds come 7.2.5. At 20% haste, we currently experience a 5 second cooldown reduction per proc on average, and this is halved to 2.5 seconds come 7.2.5.
If haste instead increased the 3 second reduction, we'd see massive increases in the amount of casts of Dire Frenzy, especially when you consider things like Bloodlust. This would just be completely inconsistent with what the developer stated.
Reducing the absolute value of the number of seconds reduced is not really a 'punishment' for having haste. The cooldown of Dire Frenzy reduces with haste, so you don't need to reduce the cooldown as much to have the same effect. As I mentioned earlier, you'll gain 0.25 uses of Dire Frenzy per Wild Call proc regardless of how much haste you have; you're not really 'losing' anything by gaining haste. (Actually it's 0.21875 uses of Dire Frenzy per proc on average, once you include the clipping issue when procs occur with very little time left on the cooldown).