Does the Siberian tiger only hunt bears when it cannot find other prey?

Only in absolute emergency, because the bear is not weak or harmless.
A tiger must be very desperate, because a hunter cannot really allow serious injuries. What brings it to hunting, but perhaps not to be able to hunt for days or weeks properly.
As an example, there are, for example, not unselfished encounters between tigers and the very resistant lip bears and the weigh up to 145 kg (Selten also sometimes up to 190) or less, with the females.
There are always confrontations with queens who bring up to 260 kg to the weighing, with females again a little less, but despite their difference in weight and mass, the tiger does not undone the shorter, because lip bears have it real in themselves, especially when the bear mother defends her boys. Here you can see how fiercely a lip bear defends, but of course it does not mean that the tiger has no chance, he also has success, but such struggles end less often deadly, usually the loser tries to pull back because serious injuries are not always worth a fight. Of course, such a fight can also end fatal and lose the bear.

https://www.youtube.com/watch?v=0ggJ627z4jU

https://www.youtube.com/watch?v=CrS7ltIsFOA