Why is the need and the value of the bias resistor is dependent to the source output resistance?
From this page: http://www.beam.org.uk/opensource/pci-6143/docsource/S_Series_Help.chm/Differential_Connections_for_Nonreferenced_or_Floating_Signal_Sources.html


  DC-Coupled, Low Source Impedance
  
  You must reference the source to AI GND. The easiest way to make this
  reference is to connect the positive side of the signal to the
  positive input of the instrumentation amplifier and connect the
  negative side of the signal to AI GND as well as to the negative input
  of the instrumentation amplifier, without using resistors. This
  connection works well for DC-coupled sources with low source impedance
  (less than 100 Ω).
  
  DC-Coupled, High Source Impedance
  
  For larger source impedances, this connection leaves the DIFF signal
  path significantly off balance. Noise that couples electrostatically
  onto the positive line does not couple onto the negative line because
  it is connected to ground. Hence, this noise appears as a DIFF-mode
  signal instead of a common-mode signal, and the instrumentation
  amplifier does not reject it. In this case, instead of directly
  connecting the negative line to AI GND, connect the negative line to
  AI GND through a resistor that is about 100 times the equivalent
  source impedance. The resistor puts the signal path nearly in balance,
  so that about the same amount of noise couples onto both connections,
  yielding better rejection of electrostatically coupled noise. This
  configuration does not load down the source (other than the very high
  input impedance of the instrumentation amplifier).


100Ω seems to be fairly arbitrary crossover between 'low' and 'high' source impedance.