In addition to what dataflow already said: while something might look free because you don't need to emit any kind of special instruction to get the desired res

In addition to what dataflow already said: while something might look free because you don't need to emit any kind of special instruction to get the desired result, doesn't mean that it's free in hardware. The reason why a lot of CPU architectures have weaker memory ordering is because this is faster on a hardware level. Consider that for sequential consistency to work on a multithreaded system, caches of all cores must be kept in sync. This is hidden from you, but does result in higher latencies and more power usage.