Coming in late on this one, but I've been on holiday.
I spent the last twenty-odd years of my working life as a technical author writing operating instructions and maintenance manuals, first for the aircraft industry, then for a company making TV camera support equipment.
All the previous posts have made valid points. There are no excuses for poor instructions - they should all be tested and evaluated before publication. However, many companies use their designers or engineers to write the manuals and they are often too close to the problems to appreciate them, and have a familiarity with the product that prevents them seeing things the way a potential customer will. But, it's cheaper than employing a technical author!
Likewise translations, which should be carried out and verified by competent nationals of the target country - just getting an English national rather than an English speaker to read and correct the translation would iron out most of the problems. As an aside, I can tell you that translation is tremendously expensive and is usually the first cost to be cut! My translation budget exceeded my salary by a considerable amount. Translating a five page ops guide into six languages cost (in 2006) about £8,000. Far cheaper to get the SEO's wife, who did French at A level, to translate it.
Having said all that, there is a defence - of sorts - for unintelligible instructions. One of the first things an author is taught is to identify his readership.This is pretty easy when you are writing for aircraft engineers or brain surgeons - it is possible to assume a common level of education or intelligence. Not so easy when dealing with modelmakers. Anything from a nine-year-old schoolboy (my first Airfix kit) to a ninety-year-old retired university professor (well, there must be one) and everything in between. Writing to suit all of them would be difficult, but not impossible. It would be expensive and would need to be recovered in the cost of the kit. There are people in this forum more qualified than I to tell us what percentages and profits would be affected, but I suspect that the price rise might be prohibitive.
Rick