The definition of professionalism is doing one’s job to the best of one’s ability within the law and ethical codes of conduct. This is embodied in many job descriptions, from teachers to doctors, accountants to park keepers. Not doing a job to the best of one’s ability is unprofessional.
Professional “codes of conduct” often spell out in detail operational and ethical principles for working as a “professional” within a particular “profession”. But in all cases, not working to one’s best abilities and efforts in a particular job would be deemed as being unprofessional.
So, now we come to “bonuses”, especially in relation to recent bonus cultures in sectors such as banking. A bonus suggests that the person is being rewarded for “going the extra mile”. But this suggests that the extra mile was somehow held back within the normal job boundaries and the normal salary. This suggests unprofessional behaviour. An investment banker goes a certain number of miles but then reveals “extra” miles that were somehow pulled out of nowhere. No one pays bonuses to a professional surgeon for “Better than average” stitching after an operation. No one gives a bonus to a nurse for showing “extra care”. Bonuses suggest that someone has to be motivated to actually work to the best of their ability, which means a bonus for being professional. Since being professional means working to the best of your ability, a bonus means the standard work was clearly unprofessional.