Remember, whether we support the dissidents or not, the tyrant can accuse the dissidents of being tools of foreigners, so lack of support doesn't deprive the tyrant of that tactic.
And it is completely wrong, as Victor Hansen reminds us of the opposition to Hitler which waxed and waned depending on how resolute foreign opposition to Hitler was. So sucking up to tyrants to make them nicer is no way to ease the tyrant out of power or get them to change their ways:
But such concessions to dictatorships only strengthen them while undermining internal dissenters. Generals of the German general staff, conservative Prussian aristocrats, and liberal reformers were all terrified of Hitler by 1939. As early as 1936, some had even sought to remove Hitler and his gang through half-baked plots. But each time the Allies backed off during an international crisis—and as Hitler added Austrian, Czech, and Polish territories to the growing Third Reich—his popularity with everyday Germans soared—and designs to remove him fizzled. After the war, early opponents of Nazism confessed that French and British appeasement had empowered Hitler and undermined their efforts.
Perceived momentum counts. The majority of the public and nations at large have no real ideology other than a wish to ally themselves with a winner. After the fall of France in June 1940, Germans mobbed the streets to catch a glimpse of their Fuhrer; yet after the Wehrmacht’s catastrophe at Stalingrad in early 1943, Hitler was afraid to speak to everyday Germans and went into virtual seclusion. Indeed, when the war went badly in mid-1944, his own military once again tried to blow him up.
Thus it has always been.
Recall when you see those "26.2" stickers on cars that the marathon race is based on the Battle of Marathon when the Athenians defeated the Persian invaders.
A runner was sent back from the battlefield to Athens to tell the people that the Athenian hoplites and their Plataean allies had won the day.
Why was it so important to send a man back for that?
Because pro-Persian elements (or rather, anti-democratic factions more willing to cut deals with the Persians) of the city were ready to throw the gates open to the Persians as the army was away. Letting the Athenian people know that Athens had defeated the Persian army deflated the pro-Persia elements and ensured that Persia could not snatch victory from the jaws of defeat by regrouping for a new battle against an army that now lacked a home base had the city defected to Persia thinking the hosts of Darius the First were on the way.
Even if the runner's tale is not historically accurate in its particulars, the point is still true. The people needed to know the outcome because success or failure against the Persians would either deflate the pro-Persian elements or embolden them.
And thus it has always been. Hitler's opposition rose and fell on Hitler's defeats and victories over his foreign foes.
We do not "taint" opposition to tyranny when we support those who seek freedom.