Yes, the employer can have good reasons and might request you for such additional insurance. However it's best to check all details first.
Let me address this question from a perspective of a person with some experience in insurance industry.
There are two factors to consider. In general you are usually liable for the property of your employer however this is usually limited to some degree. So if you leave your laptop in a car and it gets stolen you may be liable for its loss as this is in general something you should not do. On the other hand if you accidentally drop the laptop at your home it's usually insured and it the insurer takes a liability on this.
Yet the insurance has its limitations and details depends on the insurer's product and employers choice (and for large companies negotiations). If the company is based in UK and doesn't expect their employees to travel outside the country the company may buy an insurance limited to UK only (that in general will be cheaper). On the other hand they may as well have coverage for trips abroad, albeit it can be limited, e.g. to EU/Europe only or to business trips ordered by the employer only. As you can see both cases wouldn't apply if your laptop gets damaged in the US which essentially means the liability is on you then.
So yes, the employer might request you to additionally insure the laptop.
What I would suggest is to ask the employer to check details about the insurance coverage they have. They probably have some specific person responsible for that inside a company and/or a broker who is responsible for handling their insurances. Ask about two things - to check if your specific situation is covered and what should be the conditions of the insurance to match those normally covered cases. If it is covered you're clear and good to go.
If not, you should ask if it is possible to extend the insurance for the laptop and on what cost (to be covered by you as a reimbursement to your company, you'll probably need sign some extra agreement on it).
Then go to insurance company/agent and ask if it is possible to include such insurance. It can be already included (unlikely), an additionally paid extension or a separate insurance. If the insurer of your choice doesn't have such option, ask other insurers as well.
Now you have a clear situation and know the costs. It should be up to you to choose one, however check with your employer if they are happy with your choice of insurer. There may be some crappy companies as well and employer may say they won't accept such insurance.
Another approach is that you agree to cover the loss if the equipment is lost or damaged and it's not covered by the insurance. You have probably agreed to that already but if not then again - there should be some formal agreement. Now unless the agreement clearly says you are obliged to insure the equipment (two sides can agree on almost anything) it's up to your discretion if you look for some kind of insurance to cover that risk or you just accept the risk on yourself.