Well this is where you get into whether there is greater benefit to free secondary education vs possible "career" students. Also just because education is free doesn't mean living is, so a student would still need to have a part-time job. Making secondary education affordable (not even necessarily free) is a huge benefit to society, if for no other reason than those people tend to get higher paying jobs and if their income isn't tied up in paying off debts they will be stimulating the economy.
Over here, most secondary education is free-ish, certainly free compared to some of the tuition fees you guys charge over there. For the living expenses, those without rich parents get federally funded assistance through BAf?G (Bundesausbildungsf?rderungsgesundheit). The great thing is, this starts out as a huge benefit to the student - half the assistance is a gift, the other half has to be paid back interest-free at least five years after finishing with possible delays and discounts. For example, I finished my BSc in 2008 and paid back the loan half in 2013 after 2.5 years of working full time... minus a sizable discount for very good grades, and another discount for paying it all back in one lump sum.
If your studies take longer, BAf?G gets switched to a full loan with low interest rates and similar perks.
As a result, you can usually get by without a full part-time job. Do some stuff during the months off, maybe a small teaching gig during the semester, some weekend stuff... should be enough to supplement BAf?G.
The idea is that these programs pay for themselves very quickly. By reducing (not removing, sadly) the relationship between the wealth of your parents and your own education you increase the overall capabilities of your future nation/workforce. That results in loads of societal benefits, and obviously also more taxes paid by those getting a better job after uni.
Sure, not everyone who gets a full secondary education will become a better contributor to society... but those are far outweighed by the others, the averages work out well.
Not showering people out of uni with crippling debt allows them to start a family with reasonable finances, reducing the future burden on the welfare system.
- - - Updated - - -
I see your point but there are definitely times when you CAN know that you're alone. For example,
this is part of a road I used to take between my house and college all the time. The amount of traffic you see if
very unusual btw, and in that traffic you wouldn't want to go 100mph, of course. The vast majority of time I'd driven there, however, it was dead empty, with visibility for at least half a mile the entire time.
Feel free to speed then. Just don't complain when you didn't see the cop sitting in the tree line, eating donuts next to his laser gun.
No guard rails / armco stopping you from ploughing into trees when some technical bits on your car go tits up? Probably not the best place to go very fast.