As its name implies, MathJax's focus is on typesetting mathematical formulas, not text. While it can handle small amounts of text, it is not designed to process large paragraphs of text, and so does not have the full text-mode capabilities of TeX, and in particular, has no text justification ability. Because you are using MathJax within a text page, MathJax's philosophy is that you should use HTML to handle the text on the page, and MathJax for the math. You can probably produce your desired output by using an HTML <table> element or CSS that creates a table layout, with MathJax handing the mathematics in some cells, and HTML doing the text justification in others. This type of layout is outside the scope of MathJax itself.
Version 4 of MathJax (now out in beta release) does provide a means of embedding HTML within TeX expressions, and it would probably be possible to use that to produce justified entries in a cell that is of a fixed width. See the
release notes for more details (this was added in the v4 alpha release).
Davide