What we deem as ethical or unethical varies according to the person involved. Buying twitter likes can be seen as a harmless move. It is cheap, has a lot of benefits and does not harm anyone. This is a good motivation for people to use them. They have various benefits such as the potential to help one increase the number of followers. Other than that, they also help in increasing the user’s popularity on twitter. From a user’s perspective all these are benefits and do not harm anyone.
The downside to using them that bring about the question of ethics is what your followers will think of it if and when they find out. The buying of twitter likes could cause disgruntles among them especially if most of them followed your account because of the number of likes on your tweet. It is like cheating in an exam. The use may result in scam messages or other unorthodox activity on your account that raises a red flag. However some may argue that all is well as long you do not get caught.