Why can ssris (and similar medications) be considered non-addictive, yet can cause major withdrawal symptoms?

A key distinction...: Between addiction, which causes you to crave increasing amounts and lose control over your willpower, and physical dependence, which simply means that your body receives the beneficial effects of the medication and becomes unbalanced when it is gone. Some examples of medicines that your health becomes "dependent" on and you will feel unwell if you stop taking them are insulin, thyroid meds, etc.

You're right on both: Many medications have start-up side effects that go away after you've taken them for a few days, or they may have withdrawal symptoms if you stop suddenly. Stopping Clonidine suddenly can spike your blood pressure through the roof! antidepressant medications can cause temporary symptoms when you start them and again when stopping. Slowly increasing and decreasing the dose helps a lot.