To understand why citizenship-based taxation is objectionable, consider the STEM Jobs Act of 2012

Victoria Ferauge explains in as clear a way as may be possible why it is important to understand what people mean when they talk about citizenship-based taxation, and why the US effort to impose it with the big stick of FATCA is so objectionable to Americans living in other countries (also known as "immigrants" in those countries). She makes a number of points but I wanted to highlight one here:

In the [dialectic on Americans living abroad, which equates expatriation with tax evasion] it seems not to have occurred to the American public or its lawmakers that the people they are trying to attract [through immigration policies aimed at high-skilled workers] are the product of other countries' investments in producing a well-educated populace. 

In light of that, consider what the Heritage Foundation says about the STEM Jobs Act of 2012:

American businesses are struggling to fill high-skilled employment opportunities. STEM jobs grew at over three times the pace of non-STEM jobs between 2000 and 2010 and are expected to grow almost twice as fast by 2018. Business groups have spent years urging Congress to reform the visa system in order to fill these empty positions.

Enhancing the ability of high-skilled workers to enter the U.S. would be an asset to the nation’s economy as well as its indebted government. 

In case the instrumentalist nature of our interest in immigrants is not clear, the point is we're going to make these foreign-born, foreign-raised, foreign-educated, now high-skilled, highly-paid workers a permanent part of the American taxpaying public, which can increasingly be defined as the American wage-earning public. One may be forgiven, perhaps, for juxtaposing this deep thirst for foreign talent against America's ongoing disinvestment in its own social infrastructure, including public education (also helped along by the Heritage Foundation).

Victoria's post thus seems to me to contain the contours for constructing the normative case for taxing humans. There is much to be done on this topic, but it seems clear that the practice of admitting immigrants at all, let alone actively seeking highly-skilled ones, demonstrates that inbound human mobility is a social good that states support; that inbound human mobility (perhaps especially of the high-skilled kind) may be the product of rather intense social investment by some other country; and that conversely taxing on the basis of citizenship regardless of residence is a means of preventing outbound human mobility and so runs strictly counter to the overall good of human mobility while also being hostile to the aims of other countries also seeking inbound labour.

The US has long objected to onerous fiscal controls on the capital it sends across borders, so it seems particularly difficult to understand the ongoing support for placing onerous fiscal controls on its outbound labour supply. Contextualize that observation within the story of the shift of taxation from capital to labour over the past 50 years, however, and a pragmatic explanation is easily ascertained. But pragmatism does not make the normative case. In fact, it violates the conventionally-understood major normative factors for assessing taxation--efficiency, administrability, and fairness--because it introduces an insurmountable amount of arbitrariness. This topic deserves much more thinking than has been allocated to it by tax policy observers, including myself.

There is much more at the link, please do go and read Victoria's whole post.