Stocks also don’t have a “true” value. Their price is driven entirely by supply and demand, which in turn is weakly anchored by investors doing fundamental valuations on the stock (there is more demand for an underpriced stock). The issue is that market manipulation is outright theft, usually from retail investors.

Think of it like playing blackjack. When I hit, I make a bet and I know roughly what the odds are that the bet will pay out. If the house was to manipulate the cards in the deck so that the odds are different, I would lose much more often then I should, and it would be theft. Similarly, if someone uses artificial demand to drive up the price of btc above the natural demand and I overpay, then they are selling to me at an unfair price. Eventually the price with fall to the natural price and I will lose money. Btc is weird because people keep buying more, but the principle is still the same. If the price is going to go from 40k and 50k over the next few months, and the price is artificially raised to 45k (which is when I buy in this example) then even if I get out at 50k I’ve lost 5k of profits I would get if the market was fair.

So the short answer is that in any case of any market manipulation, it is theft from other investors. Usually (but not always) small retail investors.

Now the argument some crypto folks make is that market manipulation is part of this market, so take that how you will.