There is too much garbage in the Play Store (the store for android devices), one could find blatant scams there and the moderators of Google do not seem to care enough.
You need to report the apps if you want them to check whether it is a scams or not. Google seems to use a bot with a preset that allows malwares to get listed as long as it passes some surface-level check. That being said, I'd also be cautious even if you use another app store like FDroid as long as the developer does not provide a way to check whether the apk is legit or not.
I know how to check the integrity and encrypted certification of a website, not an application. So for now, it would be better to stick to the mobile web version of the most popular ones, like Stake.
It is not that difficult as far as I'm aware. Sometimes a developer provides an MD5/or other forms of a hash of the apk so you can verify it. Other times they provide the code so you can build it yourself. Just in case everyone is not yet aware, don't get fooled by SSL certificates. Some fake websites can use free SSL encryption to trick users. Make sure to double-check the address regardless if it uses SSL or not just in case you get phished. CMIIW.
And there are about 93 countries where online gambling is not prohibited. So I don't think it will be a loss deal if any casino make their android application and publishes it on the google play store as their application can be used in 93 countries, which will be a huge number of users.
It's definitely not a loss, as long as the cost is justified. Some businesses probably only have one or two countries as their customer base, so it is also understandable if they don't think an app is necessary when the website is good enough.