Yeah but if you enter the US by road the UK authorities don't know you're planning on visiting the US and can't tip them off that a convicted whatever is trying to enter. Canada doesn't care what you are so its an easier way to get in.
legally speaking the UK authorities aren't allowed to tell the US about your convictions anyway, its a breach of the privacy act (or whatever its called) but if they feel like being twats they do occasionally tell them.
Unless you're Mr Big they usually won't bother though.
When you enter the US by airtravel the US already has a file on you with like 20 something pieces of info, like how you booked your ticket, how you paid, your frequent flyer info etc. The UK can say you have a record as well, so flying into canada/mexico and driving circumvents this since neither the canadian or mexican authorities demand such ridiculous amounts of information.
Applying for a waiver in advance is just a good way of ensuring the US definitely knows you have a record. I'm in the process of changing my name anyway (hence me using two names when we chatted the other day!) so I'll definitely have no problems in future, its such an easy system to get around, but best to do it now if you want to do it, since technology is rapidly increasing and they'll one day integrate everything and be able to tell anyway.