Typically employers can, and usually do, terminate someone who is unable to work. However, it is important to check the laws of your state. If your state is an "at will" employment state then there likely is no problem with your employer's course of action. One caveat to this would be if you had a contract that stipulated otherwise.
I suggest you contact an attorney to fully explore your options.
Washington is an "at-will" state which means your employer can terminate you for any reason, or no reason at all. This would naturally include your extended absence due to disability. However, a major exception to the at-will relationship involves working with disabled individuals to try and "accommodate" their disability and return to work. Employers and Employees should work together through this process to try and find a mutually agreeable solution.
Your end of the bargain is to communicate, communicate, communicate. You should be frequently discussing your return to work plan and ability to perform light-duty with your employer. You should be asking about accommodations recommended by your doctor, and the best way to transition back.
If you have done the above, and your employer is ignoring your requests or refusing to accommodate you in any way, it may be worth contacting an attorney to discuss your rights.