17.8% of the U.S. population voted for George W. Bush in 2000 [0]. 21.0% of the U.S. population voted for Barak Obama in 2012. [1]
Seems to me not very far off from the abstract's note that "15% of all Crimeans voted" to secede. If it's legitimate in the US, why not elsewhere?
[0] Yes, Al Gore did better by winning 18.1%.
[1] Percentages calculated mainly using Wikipedia's numbers, which admittedly are not a primary source, but I'll guess are probably "close enough" to make my point:
https://ancillary-proxy.atarimworker.io?url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FUnited_States_presidential_election%2C_2000
https://ancillary-proxy.atarimworker.io?url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FUnited_States_presidential_election%2C_2012
https://ancillary-proxy.atarimworker.io?url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FDemographics_of_the_United_States
http://www.multpl.com/united-states-population/table (for 2012 population)