Candy Crush can (and does) let you play freely, only to popup the "insert coin to get help" when you are stuck. It also adjusts the game play to your level (by advancing to harder levels) so that you are always challenged and possibly get stuck. If you advance by paying, you probably will need to pay more to get further (if you failed an easier level without help you will probably fail later levels without such help).

In an imaginary world, a slot machine could have a camera to track who is playing. It would let you play for free the first few times, even give you some money, but leaving you one "advance" short of the stronger payments. After some rolls, you are prompted to insert coins to get extra advances. Of course, the machine keeps the payment structure favorable to itself, but tricking you into inserting more and more coins, all the while getting you hooked into the game. It could even adjust the payment structure to be more and more in its favor the more "hooked" it detects you are.

Regulation that prevents the above:

1. Slot machines cannot track users by any means.

2. The payment distribution has to fit certain parameters (think X% of money given back within N rolls)

Candy Crush seems to specialize in near misses, for example by creating levels where almost all of the achievements required to pass a given level are easy, and then one is next to impossible. You don’t keep $1 billion+ per year coming in on a years-old game without some serious psychological tricks.