In the US, ministers don't have to pay taxes on housing costs used in the course of providing ministerial services, whether it's free housing or an allowance. Medical expenses are not taxable for anyone. Even if you work for a for-profit company you don't pay tax on what your employer pays for your health insurance.
Same goes for hotel and plane tickets - if you travel for business for a private employer, you don't have to pay taxes on what that costs. Now if the WTS was paying for a pure vacation, it might be income. But you can bet whenever a GB member goes to Fiji, he gives a talk and thus it's the equivalent of business travel. Just one of the perks of the job.