Why are players allowed to select cards that aren't legal for their selected banlist?

This is also a way for certain OCG decks to bypass their banlist as the TCG list doesn't address those cards or to avoid other critical limits when using some OCG only cards. An example of the latter is using OCG only cards in a deck with tellarknight ptolemaeus which is now banned in OCG.

It would seem to make sense not to allow selecting cards that are OCG only cards in a TCG banlist or vice versa. This would really be no different than the limits put in place for banned, limited and semi-limited.

It would be nice if we could just play the corresponding banlist on android, but a move to shore up what people are allowed to play in their respective list would be nice.
On windows, you can choose the card pool and the banlist seperately on the official servers. I do not know, if this is different for the mobile versions, but many TCG players like to test the new OCG stuff ahead of time, so allowing a format with TCG banlist and TCG+OCG cardpool makes sense. It might produce decks, that are not legal for quite some time, or might even never be playable in an official format, if a newer banlist affects them. So it is kind of a fictional format, but it does give you an idea, how a deck, that received new OCG support might play out in the current TCG format. Which might be useful information, or just a fun experience.

Now, I understand, that there are also people, who are very serious about decks being legal according to the current real-life format, so I agree with you, that this format should be at least an option. If the seperate selections for banlist and cardpool do not exist on android, they should probably be added, in my opinion.