For Canadian citizens who enter the United States by road, it is very common not to have any evidence of their entry, and if they want to apply for another visa or adjust their status from within the US, they wonder if there is even any evidence of their legal entry and what documents they should provide to the USCIS.
This is how it works. While there is no stamp, your passport was run through a reader that downloads all the details from the passport. In other words, Department of Homeland Security (or CBP) has an entry into their database of your entry into the country. So a Canadian citizen should work with the understanding that a record of their entry exists and file the paperwork. There is no need to attach an I-94 or copy of stamp, simply indicating the date and name of border crossed.