It's not necessarily a disease but more like a drug. And just like most other drug, you can get addicted to it. And that would mean you would be spending all your time and effort to gamble which is bad for you.
A disease on the other hand is more like a disorder or a malfunction of a human part.
Yeah, it is like a disease, then, it can be easily cured with medicine. Gambling is far more than a disease; it is a big time emotional and mental disorder in a way. This is the reason why a lot of people end up going through rehabilitation at least to get out of it. Diseases can easily be gotten out from, but not something like addiction where your self-control has been fully hijacked. This is why it is necessary to be in control as much as possible even if you want to end up gambling.
Addiction is the fact or condition of being addicted to a particular substance or activity. We are all addicts! I have my rituals, everyone has it. If you like something there are high chances that you will be addicted to it if you use it or do it everyday! With strong mind you can control yourself and your activities, with weak mind you will always be week when temptation comes in any form. Weak people needs medications to makes them stronger, and there are many weak people in the world. Addiction can be a disease if someone can`t control anymore, why to not give that person some drugs? Drugs can help them to be stronger and to not think about addiction so much.