What is the block maturation time?

I was reading the wiki article about the blockchain and I didn’t understand the part in bold:

When a block becomes an orphan block, all of its valid transactions
are re-added to the pool of queued transactions and will be included
in another block. The 50 BTC reward for the orphan block will be lost,
which is why a network-enforced 100-block maturation time for
generations exists.

I don’t know the details behind block creation and acceptance, so how does the block maturation time work? Is the block only accepted after a while?

Why do so many bitcoin exchanges have their bank accounts closed and have problems establishing bank accounts?

I find it really strange that it happens that so many Bitcoin exchanges have their bank accounts closed.

Outside of the Bitcoin world, I find it very unusual to have a bank account closed, for a reason other than inactivity. The only case I remember is Julian Assange’s Swiss Postfinance.ch account closed.

Maybe it’s a specific practice of banks in certain countries?

Many banks that I had been dealing with, seemed to have the attitude that everyone with a passport (for individuals) or company papers (for companies) can just open an account, no matter what his business is – whether he is trading spaghetti, sand on the beach, air or voucher codes – it’s none of the bank’s business. Their only worry is whether the person who opens the account is really the one who he says he is. The bank doesn’t have to understand your business, their business is to verify ID and do maths. The rest is the customer’s business and responsibility. Of course, the bank has to report transactions over a certain threshold and/or suspicious to a certain government agency, but closing accounts?

Maybe it’s the matter of attitude, common practices, banking culture being different in those countries that the banks closing the accounts where established? The Bitcoin exchanges accounts are closed mostly in "old" western EU (UK, France), and the much more relaxed "no bullshit" attitudes of banks I have encountered is mostly from Central/Eastern EU banks ("new" EU countries).

Is it common for western EU banks to be so picky about customers, and closing accounts despite meeting formal ID requirements of the companies?

The sometimes mentioned claim that it might be illegal to "hold other people’s money if you are not licensed to do it" is not very plausible, as any business "holds other people’s money" for some time, and there are even ones which sell "numbers which hold a value" for money – prepaid vouchers for mobile phone recharges for example. That voucher is usually a 16-digit long number that holds some value – if that is fine, then Bitcoin must be no different – both are basically selling numbers which hold monetary value.

Why was the target block time chosen to be 10 minutes?

According to the wiki, 10 minutes was chosen as a ‘tradeoff’.

Why ten minutes specifically? It is a tradeoff chosen by Satoshi between propagation time of new blocks in large networks and the amount of work wasted due to chain splits.

However in the original Satoshi paper, 10 minutes is merely assumed for the purposes of calculating disk space requirements.

A block header with no transactions would be about 80 bytes. If we suppose blocks are
generated every 10 minutes, 80 bytes * 6 * 24 * 365 = 4.2MB per year.

Is there a discussion elsewhere that explains how the 10 minute block time was arrived at?