Because it isn't. There is nothing "spontaneous" about private property. There have been a wide variety of conceptions of property throughout history, many involving notions of collective ownership. The system as it currently exists is enforced by state violence and was created historically by privatising collectively owned property.
e.g. https://en.wikipedia.org/wiki/Enclosure
>an alternative to capitalism ... necessitates the use of coercion to enforce
So does capitalism. The only way you can "own" capital is by using violence to prevent other people trying to use it.
>Let us assume that a collective has made a decision to produce pineapples and not bananas, what happens to the guy who likes bananas and funds the production of bananas and refuses to participate in the production of pineapples?
There are a lot of assumptions here. Why is this a mutually exclusive choice? Can we not grow both pineapples and bananas? Does the man believe the amount of bananas produced will not be enough to satisfy him? How many bananas does he expect to be able to eat?
In any case, the answer depends on the system. In some systems, the guy would be punished (as he would if he were employed by a pineapple farmer under capitalism) in others, the decision of what to grow would be his, and the collective just coordinates to inform him how many bananas are already being grown nearby, so he can make an informed decision about whether bananas are the right thing to grow.
>No, the subjective theory of value does not say that, in fact it says the opposite: it is impossible to determine the value of goods since they come from the subjective choices of free individuals. Prices come pretty damn close though.
The last part is what I'm criticising. An exchange system weights the demand input to market price determination based on the wealth of the individual. This means the market optimises for the subjective values of the rich over those of the poor.
And since the rich already have plenty, what they tend to demand is more capital. This leads to a system which is very effective at creating capital, but very bad at distributing its bounties. This is perhaps good when we have a scarcity of capital, but the argument made by the author here is that we have enough now, so we should redirect the economy to distribute wealth more effectively.