Everyone on the platform is allowed to vote (or flag) with their stake however they like. Some people reserve their flags for scams/abuse, others do it because they feel the rewards that a post is getting arent justified, and still others just do it to be a dick.

If you are willing to accept upvotes from the community as judgement on what your post is worth you have to be willing to accept the flags as well. Both are just ways of people deciding how valuable your content is.