Why wasn't my passport stamped at the US border?

My friend and I (both visa nationals) were traveling from Canada to the US by bus last weekend. To my surprise, the immigration officer didn't stamp either of our passports and instead simply waved us through after scanning our documents. The entry was properly recorded, but I can't help but wonder if we didn't break some law by entering the country without a passport stamp.

What's the reason behind the lack of a stamp? Isn't it mandatory for non-Canadian visitors?