I came across this question on Yahoo! Answers:
Should M-theory read, M-hypothesis?
It being limited evidence for further investigation, perhaps not yet a theory.
I responded thus:
(I realise the words "use" and "usage" are.. ahem.. overused. Please disregard this stylistic concern.)
I believe this use of the word "theory" reflects an overlap in mathematics and theoretical physics.
In mathematics, the word "theory" is used informally to refer to "a self-consistent body of definitions, axioms, theorems, examples, and so on" (reference). Examples of this usage include "field theory" and "group theory".
This differs from the scientific use of the word "theory" to mean "an extensively tested hypothesis".
I believe the use of the word "theory" here is a purposeful reference to the fact that the inventors were interested in laying a mathematical foundation for their model.
Note that "string theory" has the exact same usage. In fact, the usage seems to be quite rampant throughout theoretical physics.
However, that's quite possibly just a way to rationalise what is actually carelessness on the part of scientists using the terms. It may not even fit with the facts; my knowledge of theoretical physics is admittedly slim.
Can anyone give any support for or against my explanation? I found this blog entry wherein commenter James Reed chalks it up to a combination of carelessness and an alternate definition of "theory" as "a model being developed by a theorist" (which is similar to the informal mathematical definition, I think).