That's a good one, I had to look it up.
"Fifty miles" is considered a singular unit of distance (made up of a more than one mile). In the same way, "human rights" is considered a singular unit "issue" (made up of more than one right).
Then I found the rule that explains it:
"Expressions of time, money, and distance are singular."
Examples:
Three hours is the time needed to complete this test.
Traveling fifty thousand miles is a long time to spend in an airplane.
Ten thousand dollars is a lot of money to spend on an education.