I have frequently pondered this in the past. The following is conjecture, but is what I believe to be true, generally:
Yes, land is prohibited to be sold to westerners in places like the Philippines and Thailand so that the people and, more accurately, the government may continue to keep political and financial control of their country. I believe that the people who make and continue to enforce these laws are quite intelligent, and have learned from hundreds of years of political and capitalist history which speaks to the fact that once a foreign entity can come in and legally own part of your country, then political influence and even control is the next inevitable step. It has a little to do with the relatively weak currency values of these countries, but money would come regardless, and buy up the country regardless. The difference between those places and a place like the USA is that there is a much smaller native middle class which can serve to buy and own parts of the country, and therefore foreign money would do much more to make and dominate the property market.
Insofar as the USA goes, yes imminent domain is an available option for the government to take your land. However, it is taboo to put into action, takes a considerable legal process to put into action, and generally the government needs a very good reason to make this happen. Although, I will allow that cases of misuse of imminent domain, due to corruption, have undoubtedly occurred. Property law in the USA is relatively strong, although not near as strong as having actual ownership of land would bring, rather than "color of title" which is what is currently bought and sold in the USA when land is transferred. The problem, as far as the banks are concerned, with having "alloidial title" (which is actual property ownership) is that a person with an alloidial title cannot mortgage the property, precisely because no one could take the land if he or she defaulted on the mortgage due to that form of property ownership being so absolute in the law.
Foreigners can and do come and buy anything in the USA that they want. I don't agree with this allowance, being a citizen, knowing the aforementioned history, and knowing the high degree of current influence that foreign governments have on the government of my country, due to both the ownership of land as well as our debt. The one thing that our government does fairly well is keeping the leverage scale balanced, and making foreign governments as dependent on us as we are on them.
I think that the reason that foreign ownership is allowed is due to the fact that there is no cohesive sense of nationality in our country based on a unified gene-pool, which leads to a leadership which really doesn't care about its people. Literally, its everyone for themselves, including our leaders. This country was founded as a slave colony and I believe, in essence, remains as such in the minds of the people in control. We are here fore the benefit of them. We are here to produce, to be bought, sold, and facilitate a little bit of buying and selling ourselves while we are here. That's the nature of capitalism, in its purest form. Any economic deference for the pure benefit of the people is absent, due to the lack of empathy, which is in turn due to lack of true nationalism. The only nationalism that we have tends to be manufactured for the benefit of certain political actions. However, its absent when we go out on the street and have little in common with our neighbors; we know that they don't care about us because they can't afford to, and therefore we learn not to care about them. The immigrants who come here tend to stick together and have much greater social networks and a sense of unity, based on their common foreign heritage, than do citizens born and raised here.
In a country like that, who would care if its sold off, on a level of fundamental morality and logic? The people who have money are going to continue leverage it, for their sole benefit, whatever that takes and whatever that means to everyone else. And they make the laws, which include the allowance of foreign property ownership. I think that they see themselves as less a part of this country than even middle class and poor people do, who live much less privileged lives within its boundaries. These wealthy individuals do not have the sense of national identity and preservation that it would take to implement these laws. Other countries, such as Thailand, may have capitalist elements in their society, but they also strike a balance between capitalism and preservation of their national identity that is fundamentally tied to their unified history as a people. In a sense, there, everyone is family.