Why does this microswitch datasheet specify a minimum load current?
The switch is available in Gold (Au) and Silver (Ag) alloy.
Gold allows switching of low voltage and current signals, such as measurements.
Silver allows switching of high voltage and current, such as power and coils.  

This difference is because the contact material degrades over time and per switching action, and a low voltage might not be able to get any current though the high resistance oxidation layer on the contacts.

This why gold plated contacts exist. (Au)
However, high current arcing destroys the gold plating, which is why Silver Nickel + Gold plating exist (AgNi + Au) to get the best of both.
When using low current, the gold remains intact, but when switching high current the gold is destroyed and the properties of a Silver Nickel contact apply.

See also the Fundamentals of Relay Technology.