Some detectorists are only interested in gold coins on the beach, so most of the junk, plus five & ten cent coins are left behind as they are being discriminated out. With discrimination being so good on detectors these days, you really can effectively weed out the good from the bad targets, so it's not that they are digging up junk and leaving it there, more like not digging it in the first place.
To be effective, you really need to work a small area, and grid it up to cover all the ground - use your foot marks as a marker of where you have been (great if the sand has recently be rained on), or drag something behind you. Checking towel marks are great, but often people will get up and shake their towels off, throwing stuff a fair way from the original spot.
When I use my Infinium, you can leave a certain amount of junk in the ground, mostly iron targets, but still have to dig other junk like aluminium and bottle caps. Best time to go detecting is in the evening after a hot day, early morning would probably be too late. Don't limit yourself to just the most popular areas, many detectorists will have likely to have already cleaned the area out.
Don't get too dejected over finding just a few coins, afterall, the Aldi detector would not be the deepest seeking detector in the world, so it is likely that you will be struggling to pickup anything beyond 5-6" down, and especially hard if coins are lying in the ground on their sides vs flat.