Because they're new to botting.
The only time an IP will get flagged is if you're mass gold farming on it.
I've gotten numerous accounts banned on my house IP, and my mule & new accounts are still safe.
Same thing with my proxies, since I only bot 1 account on them at a time.
If you get a "macro" ban, the IP isn't flagged.
If you get a "gold farming" ban, then it is flagged.