I have also encountered the same problem of the option "force y-intercept to zero" being greyed out. I believe I might have found a "solution" to the issue (though this might be something that tableau will fix in future versions). The option will be greyed out if the fields that you are plotting are not Measures (namely the columns and rows cannot be Dimension or Attribute type data).
To get around the issue, temporarily make your fields into Measures type (and tableau will automatically sum the fields by default - don't worry about that for now), add the trendline then edit the trendline. At this point, the option for "forcing y-intercept to zero" should be available for selection. Once the option is turned on, it will force the trend line to intercept at 0. At this point you can change your fields back to the desired data type. If you try to edit your trendline now, the option will be greyed out again, but the intercept should still remain at zero.
Hope this helps!