However in this case "the website" is just one of dozens of tools out in the world. 50-character summary messages are incredibly awesome across the board, and everyone should learn to write them.
I cannot disagree any more. I haven't used a terminal that can't wrap in probably 15 years. The point of a commit message is to convey information. If you could point me to a study or something showing people retain info or learn better with a 50 char limit, awesome. But until then, I stand by it being a completely arbitrary restriction.
Sorry, but your opinion is wrong and harmful. The point of the summary line is a succinct and scannable summary of the commit.
Open up any git GUI; gitk or SourceTree are my two favorite. If you keep your summary lines under 50 characters you can quickly scan a maximal number of commits. Detailed descriptions are great and should be included as well for any substantial commits, but that is not what you want in the summary. That should be added in separate paragraphs after the summary line.
This is not arbitrary, and it has nothing to do with the ability to line wrap, it's about efficient communication faced with a history of tens or hundreds of thousands of commits over time.