The Debris Will Follow You
Whilst i posted the answer about ethics first, this came to me as an after thought. Jettisoning something in space is not the same as on Earth. In space, we must remember Newton’s first law “an object in motion stays in motion unless it is acted upon” greatly applies here. In space, there is nothing to act upon the object in motion, the jettisoned nuclear waste. Whilst you may have jettisoned it and saw it drift away in the distance, that doesn't mean it has lost all momentum, it is still travelling at the same speed it was (minus the speed of your arms pushing it away).
If you were travelling at 10,000 mph and jettisoned the waste, that waste is also going to travel at 10,000 mph (minus the speed you took away by jettisoning it which would be negligible anyway). If you then accelerated to 11,000 mph, that waste is still travelling towards you at 10,000 mph.
When you eventually reach your destination and set up your colony, that waste is still following you and will one day catch up. All of a sudden your ancestors are now having to deal with the nuclear waste you left behind many years ago. Whilst the material may have decayed to the point of being non-hazardous by the time it reaches your planet, you will still have potentially hundreds of heat-resistant containers travelling towards your colony at, in this example, 10,000 mph.
Whilst this could be avoided by changing course after jettisoning or slowing down, you will have to accelerate again and use up a huge amount of fuel if you stop and start every time you jettison waste (leading to more stops and starts, causing more fuel to be used, requiring more jettisons, leading to more stops and starts etc.) Changing course would require you to at least slow down and use up some amount of fuel.
Even if it doesnt hit your planet of colonists though, by jettisoning the debris you have set a minimum speed you need to travel at. If you go any slower, the debris will catch up and hit the back of your ship. However, if you overcome all of issues these through course corrections, who’s to say the debris won’t crash into another planet like some sort of accidental orbital strike?
Although they have no legal obligation to not dump the nuclear waste, to leave highly radioactive material unchecked in the middle of nowhere is ethically wrong. It is one of the reasons why the Waste Isolation Pilot Plant has created a system to inform future generations about radioactive waste.
Explained in this answer about how to leave a message for humans 50,000 years in the future the WIPP said this:
Each component of the marking system should be made of material(s) with little intrinsic value. The destructive (or recycling) nature of people will pose a serious threat to the marking system. We decided against simple "Keep Out" messages with scary faces. Museums and private collections abound with such guardian figures removed from burial sites. These earlier warning messages did not work because the intruder knew that the burial goods were valuable. We did decide to include faces portraying horror and sickness (see Sections 3.3 and 4.5.1). Such faces would relate to the potential intruder wishing to protect himself or herself, rather than to protect a valued resource from thievery.
Here is the message that they proposed:
This place is a message… and part of a system of messages… pay attention to it!
Sending this message was important to us. We considered ourselves to be a powerful culture.
This place is not a place of honor… no highly esteemed deed is commemorated here… nothing valued is here.
What is here is dangerous and repulsive to us. This message is a warning about danger.
The danger is in a particular location… it increases toward a center… the center of danger is here… of a particular size and shape, and below us.
The danger is still present, in your time, as it was in ours.
The danger is to the body, and it can kill.
The form of the danger is an emanation of energy.
The danger is unleashed only if you substantially disturb this place physically.
This place is best shunned and left uninhabited.
Whilst we have no legal responsibility for humans 10,000 years in the future, the fact that we have developed a way to warn them about the dangers of this nuclear waste we have left behind shows we have an ethical responsibility. It would be morally wrong to leave the nuclear waste unattended and unchecked, even if it is in space.
Risk of The Unknown
Additionally, for all we know, by dumping that waste there we may damage the ecosystems of other planets, potentially ones with life on them. Would you want to be responsible for the accidental genocide (xenocide?) of alien life forms? I would hazard a guess at “probably not”.
Or there may be an intergalactic Green Peace which we’ve now angered by leaving that nuclear waste there. It would not be a good idea to start a war with an alien civilisation over littering of all things.