Well there are a host of reasons why this happens but if you ask me, it all boils down to shear level of poverty that're in African countries.
Let me use Nigeria as a case study; the minimum wage in Nigeria is approximately 50dollars/month and what most security officials don't earn anything too far off from the minimum wage. Add to that the fact that sometimes the salaries don't get paid on time and you'll begin to understand why it's necessary for them to accept/demand bribes.
There's no way a person can raise a family on a 50dollars/month even if you tripled that amount, it still wouldn't be enough because the world is changing every day and things aren't getting cheaper, they're becoming more expensive. If you have more than one child and you're earning 50 dollars a month, there's no way you can afford to send them to school, it's just not possible.
The only other option is to take bribes and thats the truth. The level of poverty is so much that these security officials often make multiple times their take home salary simply by collecting bribes from people wherever they're stationed and despite the fact that I'm strongly against it, I can't blame them.
One might argue that things like free education, healthcare and housing is provided for these people so they don't have to spend much, but then again there's more to life than just paying for rent and school fees. These people take bribes because they have no choice, or at least they believe they do. It's either they do it or their families starve and that's the plain truth.
I hope this helps.
Most security officials receive bribe in most African nations because of several reasons, below are some of the reasons;
1. They are not well paid. Most security operatives in Africa, especially the lower levels ones are not being paid much, so they have to look for other ways to augment their little pay and bribery is the fastest route most of them take
2. Because they can get away with it. When people know that they can get away with a crime, they'll continue in such crime.
3. They already have a bad reputation in the eyes of the public. So they are living to their bad reputation
4. The average African likes quick fix to things and wouldn't mind giving bribe if it would hasten the process of what they want.
5. Because there are people willing to give bribe. If no one is giving bribe, there will be no bribe to collect
1. Poverty mentality. I would say poverty but poverty isn't enough reason to betray ones morals. It's the thought that they'll always remain in that state. That's why they're always seeking for means to earn extra
2. It's readily available. A lot of people are willing to pay bribes to get away with offenses. I remember I was on a bus that was stopped by the officials. The driver was obviously in the wrong and he began to plead with a bribe in his palms. This leads to my third point
3. They feel they're helping out. Sometimes the typical officer believes that melting out the right punishment is too much for 'small' offenses. In the story above, the driver wasn't using a seatbelt. He could be taken to a mobile court for that offense and made to pay a hefty fine. Instead he pays a fraction of that fine to the officer and gets away with a warning.
It doesn't only happen in African countries, rather it happens in other third world countries including my country, Philippines.
Why they demand or accept bribes? It's simple. They are lovers of money or materialistic people. We cannot just blame everything with poverty. Yes, some do because of poverty. But if you want to live in a clean way, you won't accept any bribe no matter what your state financially is. It shows how corrupt there minds are.
It's sad that in our country, a lot of officials become active during or few months before Christmas. One example is LTO or Land Transportation Office by which the officers will become very active during the season. They will even look for a fault even you didn't do anything. It's sad that many corrupt officials still exists, but sometimes we cannot do anything about it.