This makse perfect sense if we start with property rights as a given premise. People have rights in their property and the role of law is to protect these rights; the enforcement of contracts where there has been no transfer of title doesn’t regard these rights; therefore only contracts of title transfer may be legally enforced.
But I have a suspiscion that the underlying mechanism in both property and contracts is obligation, in which case using property to justify contracts is circular.
A right creates a claim which stands opposite of an obligation. If I have a right to have my property go unmolested, then everyone else is obligated not to molest my property. Claims and obligations go hand in hand, one cannnot exist without the other. In addition to being created by rights, claims and obligations can come into existence by way of promises. If I promise to paint your fence, I obligate myself to do so, which gives you a claim. In both cases, rights and promises, we see that the claims can be waived by the claimholder, resulting in an extinguishing of the obligation. You might say “Mike, you don’t have to paint my fence,” or in the case of rights, you might waive your claim by violating someone else’s right. It’s also important to recognize that claims and obligations of promises are voluntary and exist prior to the laws and norms which enforce them, unlike other obligations which are imposed by way of force.
(Adolf Reinach developed an a priori theory of claims and obligations. Its all very Austrian. You can read more about it here.)
So the big questions are, do property rights logically precede claims and obligations, and what does this mean for title transfer theory?
The answer to the first, I think, is no. Although I said before that a right creates a claim, it isn’t really a matter of first comes the right, then it creates a claim – the claim and obligation are simply parts of what it means for a right to exist. You just can’t have property rights, either in existences or as a concept, seperate from the claims and obligations entailed in it – it would be like ignoring that a triangle has three sides. The same goes for contracts, claim and obligation is essential to what it means to be a contract.
As a result, any argument for why some contracts should be enforced over others cannot be made on grounds of property rights. The issue at hand is really, why should some claims hold legal status and have their obligations legally enforced? Appealing to property rights is begging the question because rights themselves are made of claims and obligations, and as such, are subject to the same question!
In a battle between property rights and contracts, setting aside utilitarian considerations, contracts come out on top.