It's not a morality issue, but something more along the lines of courtesy or ethics. But only if I know an honest, small businessman is vying for the same revenue. That is, if I knew someone was bidding for business and, especially if they were a "little" guy, I wouldn't try to undercut him.
I don't do it for the money, but as Kevin mentioned, I've paid for my implements and helped out some people who couldn't/wouldn't pay a big commercial landscaping company an exorbitant fee for a a non-repeat service. The commercial guys have all that overhead--insurance, training, benefits, taxes, etc. that make up their costs. Heck, it's almost impossible to make it in that business unless you have real "scale," meaning dozens of contracts. Once they have those contracts, they're typically not bothered by the little jobs we do for our neighbors.
I balance any ethical dilema by asking the neighbors if they have looked at pro services, and most have. Most have also purchased big ZTRs for the mowing, and I offer to help with the broadcasting, aerating, spraying etc. I don't ask for money and if they give me a couple hundred dollars or pay for my share of fertilizer, etc. I'll still sleep well at night.
There are also a couple gray areas that everyone should be careful about. First, legally speaking, commercial broadcasting of chemicals is probably illegal without an Applicators permit, depending on your state's laws. Second, if you damage property or hurt/kill somebody while working on a second-hand friends property, you may be up Lawyer Creek without a paddle. Third, if you are remunerated for your work, you legally have to claim the income. I'm sure the IRS and State 'Revenuers' have bigger fish to fry, but be careful how big you get. If you get big enough to be noticed by the commercial operator, don't think for a minute that they wouldn't report you to your State's Division of Revenue. Fourth, it's kind of a pain when neighbors and friends of neighbors begin to rely on you a bit too much and you can't find any golf, fishin', or porch sittin' time on weekends.
I'll stick with the onesy-twosey, good samaritan work and hopefully you'll catch me on my porch swing taking a nap or sipping a frosty one instead of finding me in a cloud if dust a mile down the road saving Joe Acquaintance a few hundred dollars. /w3tcompact/icons/wink.gif