One of the biggest advantages of Android over other mobile operating systems is the freedom users have to install different versions of the system on their phones. Since Android is Open Source, you have a plethora of community-built custom ROMs for virtually any mainstream device, and it is a blessing for experienced users who only want the good hardware but don't need the software that manufacturers offer, or for people that just want to try something new on their handset. Samsung phones are a good example of it, as many users don't like TouchWiz and choose to "flash" an AOSP (Android Open Source Project) ROM, which brings a vanilla Android experience while offering less bloatware and better performance. For example, back in the days of the Galaxy S4, switching to AOSP could free a lot of onboard memory and consume considerably less RAM. With the Galaxy S6, Samsung surprised everyone in the Android community by locking down the device, making it very difficult for anyone to install a custom ROM. If anyone out there that like flashing their devices had any hopes of installing a custom ROM on the new Galaxy S7, here come the bad news: Samsung also locked the handset down, and, this time, it will be extremely difficult for anyone to unlock it.
Every computing device has a bootloader, which is a software that is initiated right after the device is turned on, and Android smartphones tend to come with their bootloaders locked, meaning you can only install a custom ROM if you unlock it. Usually, it is not a hard task to unlock the bootloader, but Samsung has some extra tricks up their sleeves in order to prevent it. For starters, Samsung has a Secure Download option enabled, which doesn't allow any unsigned ROM to be loaded into the device. Additionally, Qualcomm gave a hand to Samsung with their Secure Boot feature on Snapdragon processors, so basically, any U.S. variant will be locked from the hardware. As you may know, the Galaxy S7 comes in two variants, one with the Exynos 8890 processor, and another with the Snapdragon 820 SoC. The latter will be available in the U.S. while the former will reach other markets worldwide. According to several discussion threads on XDA Developers, some members managed to port TWRP custom recovery for the international (Exynos) version, but there's little success on actually flashing any ROM.
It seems like all major U.S. carriers have their Galaxy S7 variants locked, which make it really difficult for users to obtain root access in order to take advantage of the full potential of Android. As usual, the community will be working hard to try to figure out a way to break Samsung's lock, and we should hear more news about the subject in the coming months as more users will have access to the devices. If you want to know in depth about the subject, you can check more technical details on the source link below.