I don't think you quite have a grasp of the company structures. Assuming that this is a large public company with a formal HR department,

1. Their first obligation is their shareholders.
2. Some companies believe that to look after their shareholders, they need to look after their employees to the best of their ability. That list of companies does not necessarily include yours.

Telling them you're leaving early will likely involve the following.

1. Your line manager being required to inform HR. Even if your manager would rather they didn't, they may have to out of job preservation. If you're manager doesn't inform HR but word gets out, it reflects badly on him/her that he didn't follow due process.
2. HR's primary role is to ensure their human resources are as efficient as they can afford it to be, (remember primary obligation is to shareholders). Your bonus is discretionary, an incentive for you to work hard next year. You can near guarantee you will not receive the $7,000 in a public company if they know your leaving. It adds no shareholder value, and regardless of how liked you are by your management, you leave them in the impossible situation of arguing that paying you is good for the company's bottom line despite you no longer being an employee.
3. You are now a security risk. You are leaving, you probably will work for a competitor as this is where you'd get the biggest bang for buck with your experience. Will you steal code? Will you steal designs?

The most likely outcome of forewarning your company is you will work your notice period, asked to ease the transition, and you'd then be asked to leave.

I have been in a similar position in the past working for a Fortune 500 company. Because of my seniority I have a three months notice period. Despite being there for over 6 years and with excellent relations its everyone, in my HR exit interview I didn't confirm where I am heading to next. That translated to them as one of,

1. I am going to work for a competitor.
2. I am harbouring ill will towards the company and plan on stealing information.

My 3 months got shortened to a week.

I don't think you quite have a grasp of the company structures. Assuming that this is a large public company with a formal HR department,

1. Their first obligation is their shareholders.
2. Some companies believe that to look after their shareholders, they need to look after their employees to the best of their ability. That list of companies does not necessarily include yours.

Telling them you're leaving early will likely involve the following.

1. Your line manager being required to inform HR. Even if your manager would rather they didn't, they may have to out of job preservation. If you're manager doesn't inform HR but word gets out, it reflects badly on him/her that he didn't follow due process.
2. HR's primary role is to ensure their human resources are as efficient as they can afford it to be, (remember primary obligation is to shareholders). Your bonus is discretionary, an incentive for you to work hard next year. You can near guarantee you will not receive the $7,000 in a public company if they know you're leaving. It adds no shareholder value, and regardless of how liked you are by your management, you leave them in the impossible situation of arguing that paying you is good for the company's bottom line despite you no longer being an employee.
3. You are now a security risk. You are leaving, you probably will work for a competitor as this is where you'd get the biggest bang for buck with your experience. Will you steal code? Will you steal designs?

The most likely outcome of forewarning your company is you will work your notice period, asked to ease the transition, and you'd then be asked to leave.

I have been in a similar position in the past working for a Fortune 500 company. Because of my seniority I have a three months notice period. Despite being there for over 6 years and with excellent relations with everyone, in my HR exit interview I didn't confirm where I am heading to next. That translated to them as one of

1. I am going to work for a competitor.
2. I am harbouring ill will towards the company and plan on stealing information.

My 3 months got shortened to a week.