Why COUNT(*) is equal to 1 without FROM clause?

In SQL Server a SELECT without a FROM clause works as though it operates against a single row table.

This is not standard SQL. Other RDBMSs provide a utility DUAL table with a single row.

So this would be treated effectively the same as

SELECT COUNT(*) AS LargeTable
FROM   DUAL 

A related Connect Item discussing

SELECT 'test'
WHERE  EXISTS (SELECT *) 

is https://connect.microsoft.com/SQLServer/feedback/details/671475/select-test-where-exists-select