While I don't think this "forces" a vegetarian diet on the employees, it is placing the employees in a situation where their company is promoting a particular ideology/lifestyle/diet through financial motivation. Dining is generally reimbursed during business travel due to the (often significant) increased cost of having to exclusively dine out, so the option for the employee is to eat vegetarian or accept additional personal finance costs. (I would also question the feasibility of finding vegetarian options all the time, but that might be due to my work travel sometimes taking me to more rural areas).
The question then becomes, is it acceptable for a business to use monetary incentive/disincentive to encourage lifestyle or ideological changes in it's employees?
In this case exacerbated by the fact that said lifestyle or ideological change involves one of the bare necessities (since all of the "equivalent" examples I've seen do not).
The bare necessity here is food in general. An employee has to eat when on business trips, so they can't a avoid the issue. They either have to accept the financial loss, or accept the dietary change.
There are of course alternatives to eating meat, but the real question is whether it's appropriate for a business to use financial disincentivis (effective pay reductions) to encourage lifestyle/idealogy change in their employees.
If the "monetary incentive" is, in other words, to provide something for free to every employee then I don't see any inherent problem. What I do think is an actual problem is making an employee pay for any potential extra expenses during a business trip.
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.
The question then becomes, is it acceptable for a business to use monetary incentive/disincentive to encourage lifestyle or ideological changes in it's employees?