*Why have they failed to take down or sanction those who painstakingly mount these adverts on the billboards?
Technically there is no outright ban on bitcoin but on transactions carrying the narration "bitcoin, cryptocurrency". This is why you can no longer sell or buy bitcoin directly from your bank.
*Why can't they simply make it a legal tender and allow more persons learn and earn from the campaigns, trades and other features that cryptocurrencies has got to offer?
As much as we want it to happen, let us face the reality - it is still a very highly volatile asset. It is like, own it at your own risk. They can not regulate it(one of the functions of government is control).
*Why does it seem legal, but illegal at same time?
It is legal because you can still use it in the country through P2P. It sounds illegal if banks get involved. Or you make a transaction via your mobile bank and write bitcoin or cryptocurrency in the narration when making paying to the other party.
*Why not pull down these billboards or sign posts?
It hasn't broken any law. Besides, it is generating revenue for the government.
*This is my own point of view sha. I'll read of others.