In one sense, I agree. It gives kidnappers an incentive to kidnap people, which is bad.
On the other hand, it gives pirates and such, who are going to attack anyway, an incentive to keep their targets alive.
And compared to the multimillion-dollar ransoms, the sums involved seem rather paltry. Would you balk at paying $100 to retrieve a hostage? $10? $1?
It seems to be a similar moral argument to bribery. Yes, it’s wrong to offer someone money to do their job, but, in some places, that’s the way of life, and you’re not going to do anything but hurt yourself by refusing to pay the bribe.