Sure you do - it's just that the costs are hidden from you in the form of commissions on profit taken by doog. Don't kid yourself - you are paying for upkeep in that you do not get a cut of gross profits, but instead you get a percentage of net.
Commission is applied to profit, not to investment. Investment does not cost me anything except microbitcoin transaction fee. Nothing compared to multi-thousand $ mining rigs.
Those (mining/investing) are two very different things, which are not incompatible, and each one carries their own risks/benefits set.
Specifically: by investing in just-dice, you get a reasonable 1% profit (aprox.), but you need to hold your money for some time in there too, and you are trusting it to a third party.
I know for sure I have more trust in dooglus than in most other "online casinos" around.