Sorry about that, this is a documentation problem. Thank you for reporting the problem. I will fix the documentation soon.
According to a developer, when there are integer constraints, ga strictly enforces bound constraints, but no longer strictly enforces linear constraints. The linear constraints become part of the penalty function that attempts to keep things feasible, but, indeed, the population can be infeasible.
As for what you can do about it, I suppose that you could give a large fitness value for infeasible members as has already been suggested. Or, since your linear constraint is particularly simple, you could write your own mutation, crossover, and creation functions that ensure both an integer-feasible and linear-feasible population, and dispense with the built-in ga version. You see, it is not so easy to satisfy general linear and integer constraints, but your constraint is not at all hard to satisfy.
Again, sorry for the erroneous information in the doc.
MATLAB mathematical toolbox documentation