In many cases consistency is preferable. Because Zee TV's graphics have been a great success for it, they are being reused for Zee Tamil and Zee Telugu. Similarly Zee Cinema's graphics are being used on Zee Cinemalu. Thus, brand uniformity is being preferred by Zee and Viacom for their channels. But don't think Zee will change Zee Kannada/Marathi/Bangla/Sarthak's logo to orange and put Zee TV's graphics on them. For one or two channels (Tamil, Telugu) this is fine, but for every channel it becomes too much.
On the other hand you have Star. Every channel has its own identity and graphics, though Star Maa's logo is too similar to Star Plus's. Even Star Pravah's graphics were slightly changed and the wordmark was made big because they were too similar to Star Jalsha. Otherwise the channels are distinct. Moreover Zee is slightly lazy (not in a negative way). It wants to reuse Zee TV's graphics on some channels (but not all), while Star wants to invest time and energy in new graphics for each channel. Each network is choosing what works best for it.