This is because their acids, such as citric acid, are weak acid and the Lemon's overall nutrients are alkaline. The End Product of Metabolism, otherwise known as Ash and it's effects on the body make it Alkalizing.

Citric Acid is a "weak" acid, meaning once it's done it's job in providing energy in metabolism it's eliminated easily via sweat and respiration. It is alkaline forming in that it stimulates the formation of calcium carbonate in the body. Calcium carbonate then neutralizes the "Strong" acids in the body, acids that can only be gotten rid of through urination, including uric acid which is the end result of protein metabolism.

In fruits such as the Lemons... the mineral content is also taken into consideration, and the Lemon's Alkaline minerals are many, calcium, magnesium, potassium, and Selenium so after the citric acid has done its job and is easily eliminated, one is left with a very Alkaline Ash (end product) from the fruit.

In a Similar way, Malic Acid (another "weak" acid) attacks Lactic Acids (A "strong" acid) making foods like Apples and it's unprocessed vinegar also an alkalizing source.

The list goes on... It's how the elements of the food, inclusive of its "acids" are utilized by the body and whether it's a weak acid or a strong acid in how it can be eliminated, and the food's nutrient content such as it's vitamins, minerals, phytochemicals, etc. that a food's ash ends up being.

Proteins... they are acidic, not only in their amino acid content, but also in it's utilization by the body... it leaves Uric Acids as it's waste product from its metabolism. And we need Citric acid to help our bodies eliminate this Acidic end product (Ash).