Guys, you all bring very good reasons "why not".
Let me give you one reason why yes - 51% attack.

It's true that
1) its risk seems highly unlikely and
2) you cannot totally prevent such event

However, by limiting single miner (or pool) to let's say 25% will make 51% attack even harder to carry out.

Having this thought in mind, what do you think can be done?

I appreciate the fact that VPN and IP proxy make this job hard, but isn't there a simple, clean, elegant way?

There are two separate questions: 1. how can it be done? 2. why it make sense to have anti-monopoly rules?

I can explain why it make sense to have location based rules.

The mining process will evolve from individual developers mining on their hardware, to could-based mining.
Eventually, large pools will take over from individual miners and transaction verification process will be concentrated in few extremely powerful computer farms. That effectively means the end of "distributed" verification. With transaction process being monopolized, its easy to manipulate the entire system. So anti-monopoly rules are not a luxury, but necessity.

To enforce fair mining some sort of cap/limit has to be introduced.
To be practical, the cap has to be very generic and easy to implement for example:
1. Cap by top level domain
2. Cap by physical location

PKI seems like an overkill. Am I wrong?

It is conceivable that a government of a state or other large entity may get into the game and monopolize the transaction validation process for non-economic motives.

Is it possible to limit the power of mining/transaction confirmation by imposing a cap based on a region or top level domain?

Thank you everybody for your responses. Your explanations were very helpful.

I am not a developer, so please forgive my ignorance.

Can double-spending be prevented by imposing rules for initiating new transactions:

RULE1: Account can only initiate 1 transaction every 60 sec
RULE2: Account cannot initiate new transaction before previous transaction is validated.