Starting with 3.0, MetadataCollector.updateLocalDeletionTime(int newLocalDeletionTime) will be called with cell.localDeletionTime() in order to update the min/max deletion times as well as the estimatedTombstoneDropTime histogram. Currently this also happens for Cell.NO_DELETION_TIME (Integer.MAX_VALUE), which causes the histogram to be called for every regular cell.
This can be easily verified by using sstablemetadata on any sstable. You should be able to find a very high value for 2147483647 in the "Estimated tombstone drop times" section.
At this point, I don't think this is causing serious harm, but could be a performance issue (see linked ticket). Calculated droppable TS times should not be affected.