When you are playing a game that is based on pure luck, you will going to still lose even if you do martingale. This is because the casinos have already setup that at the end of the day they are going to win because of the house edge advantage.
House edge is not technically an "advantage". It is the edge that the casino has due to the design of the game of chance over the player. In other words in every EV- game the odds are against the player and so even if they do whatever mind-boggling strategy they will lose at one point be it short term or long term. The casino does not have any "advantage" over its players. That would be cheating.
That is not cheating. Probably you are misunderstanding how house-edge is working, it is very much similar to having an extra advantage against your opponent. Yes, gambling houses are having extra advantage against every gambler in the name of house edge.
In normal environment, you and your opponent are having exact 50% chances for winning for any gambling. But, along with the house edge the chances go like (for example) 60% to houses and 40% to gambler. Why not we call it as an advantage to houses?
But why am not ready to call this as a cheating, because we are accepting those house-edge before playing with any gambling houses (or at least we are all aware of something which is existing). When some rules and regulations are informed well in advance, we cannot call that cheating. If we prefer and accepting those rules filled environment, we may go for playing with them otherwise we may opt for staying away from them.