While this doesn't result in a huge performance difference, and consumers can achieve the same performance by installing the bundled software to enable the OC profile, it can't be denied that this gives these cards an unfair advantage in benchmark comparisons versus cards from other manufacturers that don't use this trick.
Here's the official statement from ASUS about this practice:
"ASUS ROG Strix GeForce GTX 1080 and GTX 1070 graphics cards come with exclusive GPU Tweak II software, that provides silent, gaming, and OC modes allowing users to select a performance profile that suits their requirements. Users can apply these modes easily from within GPU Tweak II.
The press samples for the ASUS ROG Strix GeForce GTX 1080 OC and ASUS ROG Strix GeForce GTX 1070 OC cards are set to “OC Mode” by default. To save media time and effort, OC mode is enabled by default as we are well aware our graphics cards will be reviewed primarily on maximum performance. And when in OC mode, we can showcase both the maximum performance and the effectiveness of our cooling solution.
Retail products are in “Gaming Mode” by default, which allows gamers to experience the optimal balance between performance and silent operation. We encourage end-users to try GPU Tweak II and adjust between the available modes, to find the best mode according to personal needs or preferences.
For both the press samples and retail cards, all these modes can be selected through the GPU Tweak II software. There are no differences between the samples we sent out to media and the retail channels in terms of hardware and performance."