This is why ereaders should
not try to override the code specified in the book's css. The only excuse for it is the abysmal typography found in many commercial ebooks that are produced by technicians working on minimum wage. Even then, any programmer with an ounce of sense would realise that a Full Justification override should only apply to elements set to text-align: left.
Here's a blog post about the <span> oddity in iBooks:
http://www.pigsgourdsandwikis.com/20...ooks-bugs.html
Note that it's a year old ... Apple is far more interested in adding flashy features than fixing core issues, though they have at least added support for embeddable fonts.