Probably some combination of factors. Being left alone is great for the soil. The shrubbery was preventing the soil from being walked on for one, and no one was raking up the leaves and taking them away, so they were allowed to mulch the soil surface and decompose into it. Great stuff. Truly invasive plants tend to do very well in poor soil, at least the ones around here do. I am not familiar with euonymus, honeysuckle or privet as invasives, they don't do that here. Our main invasive shrub here is Scotch broom, which prefers poor, lean, dry, disturbed soils. It is a nitrogen fixer so it outcompetes most other plants on nitrogen deficient soil, and it doesn't need micorhyzal fungi so it outcompetes other plants on disturbed soils too. In some sense it could improve the soil, but it doesn't seem to - it has very little leaf litter for one, it doesn't seem to make any more nitrogen than it uses itself for another, and it keeps the soil dry by using up all the water for a third. None of the three you mention are nitrogen fixers that I know of, so the improvement they would make is probably from adding leaf litter, and the burrowing action of the roots, and preventing erosion and foot traffic on the soil. Around here, all three of these prefer soil that is good to start with, so I imagine they preferentially germinate in soil that is already good rather than improving the soil they germinated in.

Jack, I can understand why an anonymous note would make you uncomfortable, and I can also understand why you don't want to name the plant because it's irrelevant to the discussion. However, if it is on your state's noxious list, should you as a responsible citizen continue to offer it for sale? If it is not on the list then there's no issue. Here's what I would do. Print out the "Severe Threat" list for your state from the Invasives.org website Vaccinium provided, post it in a prominent location and post below it your "Company Policy" that your customers will never find one of these plants for sale at your nursery. How many of your customers have actually ever seen that list? And, how many confuse "invasive" with "aggressive spreaders or seeders"? You'll be educating them. If a customer questions the invasiveness of a plant you sell, you can show them the list and explain the difference. I have never seen and never expect to see Japanese Honeysuckle, Kudzu, Tree of Heaven, Mimosa, Common Privet, or Chinese Privet for sale at any nursery in the South. However, I have purchased Variegated Porcelain Berry and have it growing on an arbor. That plant is not on my state's invasive list, but is on "Watch List B, plants that are severe problems in surrounding states, but have not been reported in Tennessee". I'll watch it. Interestingly enough on "Watch List A: Exotic plants that naturalize and may become a problem in the future", among others are listed Buddleia, Cosmos, Rose of Sharon, Spearmint, Peppermint, and Grape Hyacinth. All of these are commonly found in every nursery and most gardens. Where do you draw the line? Do you refuse to sell any plant that is listed on not only the Severe Threat List, but also the lower threat lists the watch lists? That's your decision and your conscience.

black oil sunflower seeds will attract the most birds, but you WILL get squirrels too. You can plant native trees & shrubs instead. Do a search for native shrubs for wildlife.

I would ditch the exotics that are invasive and stick with native wildflowers. Solidago (assuming it's a native one,) is a great start. I totally agree with you that (almost) anything is better than a worthless patch of lawn that requires expensive, pollution-causing equipment to maintain and the wasting of ones' time by repeatedly using that equipment to repeatedly shorten it. It's much more rewarding to grow something than to repeatedly mangle something. Starting by learning the butterflies & birds in your area, and their preferred nectar, seed, and host plants, then trying to grow those sounds much more worthy of your time and effort than exotic invasives. And it is also not difficult to find information about natives (and some non-invasive exotics) that bees love. When plants are called invasive as a gardening term, being exotic to the location in question is assumed. All plants are native to somewhere, and when found where native, they can't be called invasive. A native can't invade its' own territory. This does not at all mean that that all or even many gardeners would want to cultivate any particular native plant. Being too big, seedy, and/or exuberantly spready is a valid reason many natives are shunned by gardeners. Calling them "wild" instead of "invasive" is more precise.