Why does foo[F[_], A](ff: F[A]) accept foo(1)?

Your Int is considered an instance of class Any (because is not a type constructor and so type hierarchy is analyzed to find a supertype that is also a type constructor), and class Any in Scala is considered a type constructor of type Nothing.

You can check this behavior with following code:

import scala.reflect.runtime.universe._

object Main {
  def foo[F[_], A](fa: F[A])(implicit ev: TypeTag[F[A]], ev2: TypeTag[A]) = {
    println(ev)
    println(ev2)
    println(ev.tpe.typeArgs)
    println()
    null
  }

  def main(args: Array[String]){
    foo(List(1))
    foo(1)
  }
}

Output:

TypeTag[List[Int]]
TypeTag[Int]
List(Int)

TypeTag[Any]
TypeTag[Nothing]
List()