Well, in the domain name instance, you recognized the value of that domain, and paid the minimal fees to register it. If you ask more than I value the domain, I'll just go get sex.cc, sex.org, or sex.xxx instead, either paying the minimal fee or paying a fair market value, depending on if it's been claimed yet or not.
Likewise, recognizing and claiming valuable land does not require much effort, but what if someone had come along and built a hotel atop an oil field? By recognizing and claiming that oil field, before it gets turned into a hotel, you save me the effort of demolishing the hotel, to say nothing of the expense of convincing the owners that it would be better served as an oil field than a hotel.
By claiming and retaining the land (or website) it it's original state, You save me the trouble of buying out owners who have invested in the site, and then re-developing it to my own taste. The Suffolk Energy Exchange might object to my building a porn site on its old domain, after all.
All of those things are merely possibilities. I could have recognized its potential and preserved it in its pristine state when others wanted to develop it, or I could have grabbed it 15 minutes before you would have after overhearing you say you were going to claim it.
Bottom line is, you had access to something. I decreased your access to that something. Now, I want you to pay me simply to restore the level of access you had before I entered the picture.
You say that if I hadn't decreased your access, someone else might have decreased it more. Well, that makes me like one of those armed gangs: forcing on you a protection service that you didn't ask for. If instead of paying me you choose to go with your second choice of land or domain, I have destroyed for you the difference in value between your first and second choice. I should be paying you for that, not the other way around.
Let's talk about domains. If there was no fee for domain name registrations, and they were permanent, I'll bet someone would have written a script to register every single domain name anyone would ever want. People who wanted to start a new website would either have to do everything he says and pay whatever he wants or be stuck using 30+ character long strings of random letters and numbers. That doesn't seem like an ideal situation for free market competition.
Instead, there's still a little bit of artificial scarcity of domain names, but even the small yearly fee keeps it from getting that out of hand. That's what the land tax would do for land. Claiming land for future sale would still be possible, just not free.