We have all grown up on a steady diet of advertising, I don't know why you find the toy troublesome, its no different then any of the following:

A fortune cookie

A comic in a Bazooka Joe gum wrapper

A piece of trivia printed on a sugar packet, oatmeal, or cereal box

The toys regularly include in snack and cereal boxes

Or for a non-food example:
In the newspaper... the comics, the crossword, the horoscope, and advice columns... none of which qualify as news, except arguably the horoscope, since in theory they are providing people very relevant and personal news.

What all of these things have in common is that they are add-ons not directly connected to the obvious purpose of the product, and they are included because more people like them enough to cover the additional cost then reject the product for the increased cost. They're not cocaine, its not sinister, its fools who think they know what's best because everyone else are stupid sheep that are the real problem, because they focus on one thing, get hysterical, and ignore all the parallel cases.

Many people like fortune cookies just enough to sway some people into eating there, many people go to one restaurant because they like the decor, neither of these examples, nor the Happy Meal Toy, represent legit add-ons to nutritious or delicious food in of themselves. Just as the crossword or the horoscope don't make my paper any more informative nor does the trivia on my sugar packet make my coffee any sweeter and God knows the whip cream and chocolate sprinkles on my mocha certainly aren't significantly increasing my caffeine intake. They just add something extra to the experience, and people like that something extra, so the producers give it to them. A car gets you from point A to point B, a radio is not necessary, but its popular and so its a standard feature, increasing the cost of a vehicle, the toy is the same, and I think everyone knows this, most of us have watched thousands of hours of commercials in our lives after all, you don't really think any parents thought McD's threw in toys for altruism or didn't think a kid who enjoyed the toy might be more inclined to want another Happy Meal in the future, did you?

There are literally thousands if not millions of examples of these tiny little lures used to promote items, the only excuse to go after fast food on this while ignoring everyone else is that its bad for your health, but as Chas just pointed out, citing an article an incredibly clever fellow on this site once posted, fast food as particularly unhealthy is basically nonsense, thus removing even that incredibly thin justification. I don't think 'coco-puffs with secret treasure inside' ever fooled many adults for its health value, and as we say, if it did fool them, then the kids under their parentage have much more serious hurdles for their childhood then a slightly increased risk of cavities and obesity, like Dad's bad habit of blowing the family savings on bridges people keep selling him for low low prices.