I'm not sure why you have posted this in the Complaints forum nor whether you are asking if it is a requirement set by the the Tibetan authorities or whether it is advisable as the better choice. I have no direct information but would suggest that in making your decision about whether to travel to Tibet or not you bear the following in mind:

1) The FCO website states:

"Tibet and the Tibet Autonomous Region (TAR)You can only travel to the TAR on an organised tour and you must get a permit first, through a specialised travel agent in China. Chinese authorities sometimes stop issuing these without notice, and also restrict travel to Tibetan Autonomous Prefectures in neighbouring Provinces, even if you have a permit. You should check with tour operators or travel agents and monitor this travel advice and other media for information.Once in Tibet you should avoid demonstrations and other large public gatherings. Ongoing political and ethnic tensions can lead to unrest and protest, sometimes violent. Security measures will be tight and unauthorised gatherings may be dispersed by force. Don’t film or photograph any such activities or outbreaks of violence. Local authorities will react negatively if you’re found carrying letters or packages from Tibetan nationals to be posted in other countries.Photography in Buddhist monasteries requires permission and carries a fee." So it would appear that you do indeed have to book to be part of an organised troup group and you do have to obtain your permit from a specialised agency in China first. In addition it looks as if you have to enter Tibet via China with at least one overnight stay in China before entering Tibet.

2) Despite being described as an 'automonous region' Tibet is ipso facto a region of China that is tightly controlled from Beijing and a very tendacious one at that, so it doesn't surprise me that holiday visitors/tourists are only allowed in if they are part of an organised tour group and you should expect to find what you can and can't do very tightly controlled. Equally from your point of view, Tibet is probably not a country where you want to antagonise the authorities by attempting to step outside of the boundaries they set.

3) When choosing between booking with a local travel agency and one from outside the country I am visiting, my preference is always where possible to use the local outfit because a higher proportion of the money you are spending is more likely to remain within the country you are visiting rather than being syphoned off back to another country. However, in Tibet the ethics are probably not quite so clear cut because 'local travel agency' almost certainly means one run and controlled by and for the Chinese authorities.

Thank you very much.