I agree. I think there is only really an issue when it is a monetary 'disincentive', i.e. it's a situation where the employee is effectively being paid less then what was expected based on the work-pay agreement (both in terms of actual contract and social/industry norms). If it's a monetary 'incentive' then it's adding something in addition to what was expected.

Also, it's an even bigger issue when the scale of the disincentive is largely a result of decisions outside the employees control. An employee often has limited control over when and how long a business trip may last.

Basically, encouraging people with an additional 'perk' seems reasonable to me, but encouraging them with what amounts to a 'fine' is not.