Ketones in smaller quantities are perfectly normal. Everyone has them at some point during the day, whether D or non-D, high carber or low carber. You make them whenever you burn fat. The most pronounced time is after fasting. Once the body has stored excess carbs as fat (via insulin, injected or self-produced), that fat can then be accessed whenever it's needed for fuel. Ketones result.
The only difference for low carbers is that they'll be eating fewer carbs > using less insulin > using more fat for fuel and will generate modestly higher ketones, but nowhere near the levels required to alter blood pH. The ketones themselves are not a major fuel source, though the brain and some other tissues can use them in exigent circumstances.
Even low carb brains run mostly/partly on glucose under normal circumstances (as opposed to running off of just ketones), but it need not come from carbohydrate intake. Protein (and to a much lesser extent fat) can be converted to glucose so that the brain, red blood cells, and other tissues that need glucose for full function can get it. As long as enough is generated for critical functions, all is well. It's only when insulin levels fall to critically low levels (like in untreated D) that metabolism shifts too far and starts generating too many ketones > acidosis > very bad things.
The body is a beautiful machine. It was designed to not waste a drop of energy. It can survive on what's available, whether it's carbs or protein, veggies or meat. Anything extra, it's saved for later (which, maybe, is why losing weight can be so difficult). What you fill your fuel tank with is a personal choice...I don't think there's a one-size-fits-all answer.