"Two threes are six"
"Four fours are sixteen"
when talking about multiplication tables. We also say
"One ones are one"
But one isn't plural, so we should say "One one is one", right?
Yes, one should say, "one one is one" ... and "one four is four" and "one n is n". Even "One zero is zero." Nothing funny going on here. As per Strunk and White in The Elements of Style, The number of the subject determines the number of the verb.
Note (revised based on comment by @AmanThakker): "Two threes are six" could be either multiplication or addition. "Four fours are sixteen" seems like multiplication but it, too, could be addition. If one were to say, "Three twos are six" or "Two eights are sixteen," most readers would infer addition immediately. Obvious but perhaps worth noting: our minds fill in the "operation" blanks one way or another almost automatically.