You also should be loyal to the company you applied to work for. Users that leave a campaign because they see another one start that offers .002btc per week more are not users ill be looking to keep hiring. If you apply to a campaign and are accepted, you should try to stick around til you're paid for that week. Then it's acceptable to leave.
If you plan on leaving a campaign, you should either post in the campaign thread you're leaving, or send the manager a message out of respect.
This is the common thing that happens most of the time in my opinion. It almost happen to me as well, well as showing of respect i really do agree that we should tell the manager that we will be leaving the campaign. Yet most of us do quit even if the round does not end yet(not to mention when some big company throw a campaign service last few weeks)