Understanding an advisor’s refusal of an internship
If you consider the Master's assistant position from the perspective of a paid job, it would be unreasonable to expect an employer to grant what essentially amounts to a sabbatical. Professors and school teachers are often given sabbatical leave, but Master's students are in a very temporary position. It's not a common thing, and there are no long-term benefits to the employer to outweigh the inconvenience.

If it is feasible to do so within the bounds of the research project, then it would certainly be reasonable for the professor to grant the student's request. But I don't see that the professor has an ethical obligation to do so.

Funded research is commonly held to tight deadlines – once the timeline of a project has finished, any unspent money usually returns to the funding organization.

If the professor could do so, without a negative impact on the research, I am sure that any reasonable person would grant the request for leave. The simple answer is that it's quite likely that he or she simply cannot feasibly put the research "on hold".