Re: Blame Canada
I do it every few weeks on average. Brit living in Vancouver BC, travelling to the states via road, rail and air.
If you have a Canadian or US passport there is no need for a visa or visa-waiver. You must present yourself at the border when you enter your destination country but you're not specifically 'checked out' of the country you departed from. This has always struck me as slightly odd.
If you're a non-Canadian or a non-US citizen then you will either need a full-on visa or a visa-waiver (in my case). The visa-waivers cost about $6USD and expire after 3 months. During that 3 month period you can come and go as often as you like for purposes of tourism (not work/study) without needing to renew the visa-waiver.
BUT... It's MY responsibility to insist to the DHS guy in the US booth that I need to park my car, come into the office and pay for the visa-waiver (plus give my fingerprints AGAIN and have my photo taken AGAIN.) They often won't look for an existing visa-waiver stapled into my passport and, the first time I crossed, they just asked me where I lived, to which I responded "Vancouver". The DHS guard made an assumption and waved me through with my Canadian wife. Then several months later I got a bollocking from a different DHS guard who said that it was not their responsibility to ensure that I had the requisite paperwork/permission to enter the country and that I should have surrendered myself to apply for the visa-waiver and that they were considering rendering me inadmissible to the US for the next few thousand years.
To complicate things, I now have a Nexus card (trusted traveller program) which means that I can be pre-cleared for most security crossings into the US as long as I travel with my UK passport and Nexus card. However, I am STILL required to have a visa-waiver and therefore now have an even tougher time convincing the border officer that I need to come into the office for processing. (For those who might not know, a Nexus card can cut down a border wait from over 2 hours to less than 10 minutes during peak periods).
However, to complicate things further, if I arrive into the US by air, then I need to pre-buy a 2-year ESTA that is electronically attached to my passport that allows me to land in the US when I arrive by means other than land/sea.
They MIGHT be able to make it more complicated and convoluted and prone to error, but they'd need to work hard at it.
Friends here in Vancouver - especially those who live in the towns and villages which press up against the border - fondly remember when they were kids on bikes in the summertime, being allowed to cycle across the border with ZERO ID whatsoever and only stopping to show the DHS and CBSA guards the fish they'd caught while they were down there playing for the day.
Google "Point Roberts" for an example of a piece of US soil that is not actually attached to the contiguous USA but can only be reached via Canada. Kind of like if Cornwall was part of France. There's a small border crossing there that we use when we want to buy cheap gas or pick up a parcel that couldn't be shipped to Canada.