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?
It sounds like you entered the US under automatic revalidation.
Under automatic revalidation, people who have been admitted to the US in a non-immigrant status can travel to Canada or Mexico (and, for F and J visa holders and VWP participants, various Caribbean islands) for up to 30 days and return to the US under the same admission that they had previously entered under.
This is the so-called 90 day rule that people mention when talking about visiting these places from the US: Because you are admitted under your previous entry, you don't get a new 90 day admission; instead, your previous 90-day admission still applies and you must depart the US before that date.
