To state the obvious:
An employee who steals from a capitalist (be it product or money) is a criminal (has violated criminal law).
A capitalist who extracts surplus value from her employees is just a capitalist.
Many of us do not see a great deal of difference between these two acts, society as a whole sees a large difference but many Marxists do not.
Even though both acts involve appropriating the property (labor power) of another to further one's own material well being, there is an important distinction beyond the obvious criminal aspects:
A capitalist who finds a way to further exploit labor is rewarded with more SV to appropriate and distribute (and ultimatly more profit), a furthering of the capitalist's own well being.
A worker who becomes increasingly successful in workplace theft certainly gains more as an individual, there is however another result as well. Workers increasing their rate of theft from the capitalist will result in the capitalist having less SV. A fall in surplus value for the capitalist may just result in a lowering of profit more likely if the worker is successful enough in their theft it will have some impact on the capitalist's ability to secure the conditions of existence of their enterprise (the most important use for SV for the capitalist), the worker's theft will force the capitalist to take action.
Again holding aside the criminal aspect of worker theft (or assuming the capitalist does not know about the theft, or at least does not know which worker(s) are stealing), the lower SV coming into the corporation will result in the capitalist needing to raise the rate of exploitation to continue to secure existence. Assuming the firm in question needs a certain level of surplus to continue to operate a worker that is successful in stealing will result in the capitalist needing* to further exploit all workers. Wages and benefits will be reduced, hours and intensity increased, etc.
This issue is more complex than I am presenting here, but the argument can be made that a capitalist stealing more labor power helps the capitalist, and a worker stealing back some of the surplus that has been taken from them does nothing more than cause the capitalist to further exploit the workers. On all levels the system is set so that the worker just cannot win at this game.