This protocol causes a valid and enforceable contract to be formed.
In fact, the constraints imposed by the protocol are almost exactly what you might learn about contracts in the first year of law school. A contract is composed of a 1) reasonably specific offer, 2) acceptance of that offer, and 3) some consideration between the parties.
By forbidding vague offers, PG is assuring that obviously questionable or unenforceable agreements aren't made. The consideration in this case is the startup reserving space in its round for the investor.
As far as steps 3 and 4, documenting the agreement is obviously valuable, but the contract is formed at the end of step 2 [2].
Now, it does seem unlikely that anyone would try to enforce this in court, in the same way that few are going to start a legal case over someone backing out on a term sheet, but if you could show damages based on your reliance on the other party's performance, you would in theory have a case.
Interestingly, it seems that either PG et al. must have aligned this protocol with the constraints of contract law, or in trying to achieve their ends, they independently reinvented the contract formation protocol that has been with us for at least hundreds of years.
--
Edit #1: Regarding the questions along the lines of, "this can't be a contract because there are many other terms to address," a contract can always be longer. If you don't address a term in a contract a court will try to divine the intent of the parties, or look at industry norms, or use defaults established by statute law, and try to do something reasonable. A valid contract isn't dependent on covering every possible, or even every usual, term. The fact that one party reasonably relied on the agreement and was thereby harmed by the other party's non-performance is often going to be sufficient.
[2] (edit): By reducing the contract to writing, even just one sentence in an email (steps 3 & 4), you would fulfill many statutory requirements for written agreements on certain kinds of transactions.
Edit #3: To be clear, I believe the contractual nature of this protocol is a feature, not a bug. You really do want the elements of a contract regardless of whether this is going to be enforced by a court, a person's own conscience, reputation networks, or public shaming.