To try and shorten my response - I agree with you that the core issue is a predatory design, and that the addictions have differences to physical addictions.
However I still disagree that putting a $50 (for arguments sake) cap on the amount to be spent will even help. If that issue worked, it would work for alcohol too. Just because alcohol makes you ill doesn't stop people with an addiction from drinking themselves sick and waking up and immediately resuming drinking. Coffee may have been a bad example but tobacco is another reasonable comparison - limiting tobacco consumption to an arbitrary limit won't help people with a nicotine addiction.
There are many games out there (world of Warcraft for example) which impose weekly lockouts - no matter how much you play you can't progress past a certain point. What ends yo happening is people start at the lockout, burn through the allotted content as soon as possible, and then just wait a week and come back. Implementing a limit isn't going to stop people who are addicted to these games from finding 5-6-7 of them to make progress on to scratch that itch.
To address one of your claims directly:
> The proposal doesn't restrict the fun-giving part, only the addiction-inducing and addiction-exploiting parts.
Firstly how do you define which parts are fun giving and which parts are addiction inducing? There is a very fine line between engaging and enabling, and it's a fuzzy one. At their core, the majority of games are Skinner boxes, and everything else is dressing over that (even MP games). What is firmly in the camp of fun for many many people could easily be considered enabling for a large group of others. People enjoy progression systems in games, they are engaging and fun, and there are clear cases where these are abusive. I don't believe that saying "if you can get 20x more people addicted you can still earn the same" is the right way to punish those abuses though.
We should look at ways of reducing the reasons why people fall I to these traps, and look at regulating the monetisation methods of these games, otherwise they'll just change tack and appear in a slightly different form in 18 months time with creative workarounds (which have already been mentioned - multiple games with tie ins, gifting, streaks, external purchased to interact with the game)