I think the ethical perspective, which is what you are asking, is pretty much covered by others answers; I agree that it is not much of a problem.
However, your question also implies a legal consequence. And here things get more complex, as this differs from country to country.
I have noticed in other answer on Workplace.SE before that many other countries seem to be pretty relaxed by doing something like that. But as also other people are looking for advice here, I would strongly suggest not to use bonus programs for your own personal use if you are based in Germany. I can't speak for other countries, but it might also apply to other countries.
German employment law is very strict about theft, even for little sums. In Germany, using a bonus program for company expenses on your personal card can at least get you a written warning (after 3 written warnings you are fired). You can also immediately get fired, but this depends on the circumstances.
To back this up, here is a case where such an incident occurred As was pointed out in the comments, the following case is a bit different as the employee was employed at the gas station, so he used customer bonus points for his own usage (see last paragraph for another example). Here us the article (in German): http://www.ra-herren.de/?p=1338
The employee used the payback card (basically a reward card, you collect points and get small gifts and such) when filling up the gas tank. So basically the value of the bonus points for a full tank of gas is certainly well below one euro. Even then, the employer fired him immediately. However, the court ruling in this case decided that there was no case to fire the employee, instead a written warning (which is also quite serious, as said before) should have been issued. The reason in this case that he wasn't immediately fired is that the employer did not specifically told him the handling of such bonus cards - However, the court ruling says that this was a "severe incident".
Of course, 99% of the employers will not care about that - But if your employer wants to get rid of you, he could always use this against you (even if it occurred well in the past, so even if you are on good terms with your employer now, this is not guaranteed) to last forever.
As was pointed out in the comment, my example above is not that much comparable. Another example in Germany would be the Bonusmeilenaffäre, where politicians used bonus miles (collected through their official business trips) for private flights. This was frowned upon and quite a few high-profile politicians had to resign. Note, that this is a bit different, because politicians do not have an employer.
Another example would be case, where a nurse was fired for "stealing" 6 Maultaschen (a regional dish, pasta with a filling, 6 pieces is roughly one meal), which would have been thrown away otherwise.