One of the key differences between museums in the UK & those in the US, is that while the ones in Britain tend to often be run almost as though they are a distant offshoot of the government, those in the USA tend to be run far more like a normal business.
In the UK, deaccessioning of any form is generally seen as something to be avoided – the charters that govern many of the countries major institutions explicitly prohibit it except in a very narrow range of special cases. This tends to lead (whatever the intention of the institution) to the dominance of quantity over quality, meaning that maintaining the overall quality of the collection is only possible by keeping vast amounts of it permanently in storage. In the USA on the other hand, the opposite approach is often taken. The Kimbell Art Museum in Fort Worth, Texas is an excellent example of this approach, whereby acquisitions are only ever made with the aim of enhancing the quality of the collection & as a result are generally countered with a consequent deaccessioning to remove one of the less significant pieces from the museum. As a result, the museum’s collection is generally percieved as gaining in quality over time, rather than merely increasing in size or scope. The running costs of the museum are also significantly reduced by the fact that it does not need to create vast stores of artefacts that are never seen by the public except at special request.
More contentious though in the US (& even more so in the UK, particularly in the case of the Watts Gallery ), is the idea that museums could sell off parts of their collection to cover their own operating costs.
The problem is that whilst there are benefits to both arguments, many institutions in the UK use anti-deaccessioning clauses in their governing charters as something to shelter behind when restitution requests are made, rather than actually dealing with the issue itself.
New York Times blogs 
September 14, 2010, 4:38 pm
Board of Regents Ending Injunction Against Museums’ Art Sales
By ROBIN POGREBIN
In a surprise development in the battle over whether museums should be allowed to sell art to cover operating costs, the New York State Board of Regents on Tuesday approved the expiration of emergency regulations regarding such “deaccessioning” on Oct. 8.
Those rules, which enjoined such sales, have been in effect since 2008. After hearing views from museums statewide, “there was no consensus on the efficacy of those emergency regulations,” David Steiner, the state’s education commissioner, said in a statement. Thus, “those regulations will be allowed to expire, allowing the prior regulations regarding museum collections to once again take effect.”
Last month the board indicated it planned to make the emergency regulations permanent, in part because a bill to prohibit cultural institutions from selling pieces from their collections to pay for expenses had stalled in the Legislature. “This removes a substantial obstacle to the monetization of art held in the public trust and to the transfer of art from public hands to private hands,” said Assemblyman Richard L. Brodsky, who led the drafting of the bill. The education department also said it was developing an advisory group to inform the Regents’ future decisions on collections and other museum matters.