We went to Nong Khai today, to try to go to Vientienne. We were told that "Laos is closed today". The message was very funny, because countries can't close. Translations are often humerous, both ways. I sometimes speak Thai with English grammer, which always gets a smile or a laugh. Very funny, dumb foreigner thinks he can speak Thai............LOL
Very, very few countries ever close their borders. America has never closed its borders and most of Europe has been 'open' since the end of WW2. France and the U.S. are on opposite sides on nearly every international issue, and have had very important dignitaries from every major country but still kept their borders open to each other and all other visitors. Some, very few totalitarian countries close their borders for "internal security" reasons. Often, it is so police forces can be concentrated to ensure there are no protests or riots by their own people.
South East Asia is odd about opening their borders. Only one country has kept its borders open during the last century and that is Thailand. Even with serious Moslem unrest in the south, Thailand's border with Malaya is still open. Trains to Singapore leave Bangkok every day. Freedom speaks its own language.
Burma has a repressive, authoritorian, Communist government. Even though there is a railroad line to Bangkok, they are afraid to open their borders. They saw what happened to the U.S.S.R., so they are afraid of losing power over their own people. Cambodia is the same way, plus they are extremely sensitive to comments about history. If they truly believed the land was theirs, they wouldn't care what the world (or Thailand) was saying.
That concept has been called the "Power of Right" and supercedes the "Power of Might". The power of right shows us that the truth prevails. A liar must try again and again to prove his point, but a person who tells the truth doesn't care. The world's record of history has given Thailand the "Power of Right". An excellent example is when a celebrity said that Angkor is rightfully Thai, the world knows the truth and agrees, but Cambodia, also knowing it is the truth, had to generate protests and riots, in a vain attempt to change public opinion. It just made them look foolish.
If Ayuttaya had not helped them in the 14th century, Cambodia would have disappeared and the world would not know them. The world understands this, but Cambodia is incensed that they owe their entire existence and record of history to Thailand.
Sometimes Laos, especially Vientiane is closed for security reasons. At anytime, the Lao government can stop foreigners from entering Vientiane when we have a high profile guest such as a President, Prime Minister etc visiting from another country. I can confirm that there was a Prime Minister, some Foreign Ministers and other officials having an important meeting in Vientiane on the day you tried to enter. The closure is to stop trouble makers entering such as the Hmong.
The reason why countries like America, Thailand and most of Europe has never closed its borders is because they are big countries and they are also more developed than Laos. They have enough police force, military personnel, security and emergency services to respond whenever needed. The countries that you mentioned don't close their borders, but I'm sure they close certain areas when there is an important person in the vicinity.
The celebrity you are talking about is a Thai actress, so being Thai obviously she thinks that Angkor Wat belongs to Thailand. I disagree with her, the truth is that Angkor Wat first belonged to Cambodia because their ancestors (the Khmers) built it. "Angkor" isn't even a Thai name, it's a Cambodian (Khmer) name. You are right that the Cambodians are sensitive to comments about their history because they suffered alot.