Why are classes and URIs case sensitivity?

After a discussion with +Aaron Bradley I had a question I thought was worth
posting here...

Why is uppercasing the first letter of each (subsequent) word being used
for classes and URIs?

Now this question has an SEO origin, since it's considered bad practice to
have URLs return different content depending on upper- and/or lowercasing
(part of) the URL. I was wondering why classes and URIs deviate from this.

Now I bet there are good reasons for this but I can't find a clear
description about the 'why' and hoped one of you could clarify this for me.

Thanks in advance,
Jarno van Driel