Why is it unacceptable? Well i agree that it is an inconvience, I dont believe it is unacceptable.
Flyglobespan is going to do what is more profitable in the long run. The winter session is obviously much quiter and as such the amount of passengers per week doesn't justfy the requirement for a 767, it just isn't economical.
It's far better for the airline to lease there 767's out over the quiet winter session and operate the flight with a smaller aircraft to justify the passenger numbers.
You can't really complain. Im sure somewhere on your confrimation or the website, you are told your aircraft MAY have to tech stop.
Regards,