Why do you wait a year to fertilize trees/ shrubs?

I've read that I should not fertilize trees and shrubs the first year. Is this true? If so, what is the reasoning behind this? We would like to make sure we do the right thing for our new shrubs and trees that we planted a few months ago. Thank you in advance.

Theoretically, to give them a chance to get established. Whether that's true or not, who knows? I've found over the years that many of these "rules" have no basis in fact. This one probably came from the discovery (finally) that improving the soil in the planting hole kept the roots right there instead of spreading. Once they are in and growing, I think it's safe to ignore that "rule".

The rule of thumb is to prepare the planting area well with either well rotted manure or, home made compost, or, bought compost from the garden store and you add some plant feed to this new soil before you plant the trees, then you leave the trees alone to get their roots established while the goodness you have added to the soil will feed the roots, hold some moisture and air in the soil to give your new trees or shrubs the best chance possible to thrive their first winter / summer /cold / wind etc. all that the grower requires is to make sure there is enough water getting to the root area, to help, depending on climate you may need to make funnels out of plastic bottles so you can fill them up from the hose pipe to get water to the roots in very hot, dry weather.

Therefore what you have been told is correct, the preparation is done before you plant, this eliminates the need to have any treatment (fertiliasation) for the first year after planting.

IF you did not prepare the growing area before you planted, then there is no law telling you not to add fertilisers, feeds, etc, but you must make sure this stuff does not touch the trunk, leaves etc and make sure you rake this well into the soil, don't add more than recommended on the packet and water it in well so it gets down to the roots and it cant sit on the surface of the soil.
IF you add feed / fertilisers this late in the season, you could encourage new growth that will not be hardy enough to withstand the colder winter season, this could be killed off and allow decease to enter the dead or dying tender growth and MAY kill off the tree / shrub and you could loose them all-together.

What you have been given by way or rules, guidelines, etc, is correct in most cases, but what I would say is, these are well tested and over many years been proving to be correct for most areas etc, but you have to try out your own methods to know what you can or cant to if you follow what the right or wrong way is FOR you.
Hope this lets you understand a bit more about what is required AS A Rule, and you can decide for yourself the best way for you. All I can say is that most good practices have came down via best knowledge of doing and growing, so this is not all old wives tales but practiced over many years (hundreds) and handed down to other gardeners growers who could examine the proof after the plants have done well.