Please explain why Busy Waiting is generally frowned upon whereas Spinning is often seen as okay. As far as I can tell, they both loop infinitely until some condition is met.
Answer
A spin-lock is usually used when there is low contention for the resource and the CPU will therefore only make a few iterations before it can move on to do productive work. However, library implementations of locking functionality often use a spin-lock followed by a regular lock. The regular lock is used if the resource cannot be acquired in a reasonable time-frame. This is done to reduce the overhead with context switches in settings where locks usually are quickly obtained.
The term busy-waiting tends to mean that you are willing to spin and wait for a change in a hardware register or a memory location. The term does not necessarily mean locking, but it does imply waiting in a tight loop, repeatedly probing for a change.
You may want to use busy-waiting in order to detect some kind of change in the environment that you want to respond to immediately. So a spin-lock is implemented using busy-waiting. Busy-waiting is useful in any situation where a very low latency response is more important than wasting CPU cycles (like in some types of embedded programming).
Related to this are the terms «lock-free» and «wait-free»:
So-called lock-free algorithms tend to use tight busy-waiting with a CAS instruction, but the contention is in ordinary situations so low that the CPU usually have to iterate only a few times.
So-called wait-free algorithms don't do any busy-waiting at all.
(Please note that «lock-free» and «wait-free» is used slightly differently in academic contexts, see Wikipedia's article on Non-blocking algorithms.)