I have no problem with kids missing a few days here or there, if the parents intend to teach them everything they missed while they were gone. Yes, learning takes place outside of the classroom. Unless your vacations are significantly different than mine though, trips to see world cup or visit disney or walk down the grand canyon don't typically contain lessons on solving equations with variables on both sides or how to diagram a sentence. I want to be able to just give the kid the homework and say, "Here you go, know how to do all this when you return."
The problem is, that's not what has EVER happened. The parents want the notes ahead of time, they want specific links to online resources for their child to use, they want their kid to stay after school every day with the teacher the week before and after they are gone to catch up, etc. They basically want free private tutoring. They pull their kid out of the schooling system THEY CHOSE, and then expect that same schooling system to treat their child differently. If you don't want to play by public school rules, don't send your kid to public school. Homeschool, private school, community college courses, or something with more flexibility.
Selfishly, every time a child is absent for an entire week, I'd say I have an extra 6-8 hours of work to catch them up. I should bill out at the going rate for an algebra tutor ($75/hour) to catch those kids up, and see if the parents still want to skip class for such a long period of time. "Oh, you're going on a cruise the week before spring break? Awesome, that will be $500 to catch your child up if they aren't where they need to be when they get back".
Maybe then those cheaper plane tickets wouldn't look quite as tempting. Italy's not going anywhere. It will still be there next summer, or after the kid graduates.