Weeds do come in with newly landscaped plants, and instead of complaining about it, more sensible to just keep on top of pulling them. It is difficult to mulch so completely that weeds don't have a chance, especially when you have areas of ground cover between pavers. I think it is very unrealistic to think that a new garden should be weed free, and this part of the gardening is now on you, unless you contracted to have the installer maintain the new garden.

If you knew you previously had the yellow flowering Oxalis pes-capre in the old garden, you should only have expected it to remain. If it was introduced within the root balls of the new plantings, it is probably easiest to contain this infestation if you dig the plants back up and carefully comb through the roots to remove any trace of the tubers/roots. This Oxalis only gets worse over time if not watched. The fact that it is coming up both in bare spots free of plants and within new plants makes it more likely that you already had it, or it was brought in as infested top soil/mulch, if you never had it previously.

The other purplish foliaged Oxalis with the much smaller yellow flowers is a common weed in gardens and nursery pots, but is much easier to control just by keeping on top of it, and not leaving any bare earth showing.

I have seen Oxalis pop landscaping fabric. You really should remove all the Oxalis by digging out the roots and this process takes a while before applying the mulch. Now you have to go back and dig it out. If it came with the soil you have the right to complain or a get a refund or something. I saw someone with a ruined patio, thankfully it was a rental unit. The Oxalis was growing it the cracks of the patio concreate. There was no way to dig up the Oxalis without removing all the concreate with a Jack hammer and starting again. They probably had the oxalis to begin with and decided to concrete it over, but over time the concrete cracked the oxalis came back, or else it found some way to germinate in the cracks. Weed killer won't take care of it, because it won't kill the root. Maybe if one put it on a constant program of round up over and over forever it could be controlled. I don't know. I just dug mine out, when we got the house. The roots have these small white round balls, and you have to get the balls out or that is part that will come back.

Weeds are part of gardening. They come from many sources. They might have been in the nursery plants, but if they weren't showing foliage, how would anyone know they were there. Seeds could have also been blown in with the wind, carried in by foot traffic (human or animal), or just have been lying dormant in the soil until is was disturbed. There is also the "poop factor" from birds to consider.
I think it's really unrealistic to lay blame on any one source and as was said, it really is just up to you to stay on top of them.

A really neat tip I learnt from my neighbor's gardner is to take a sharp edged tool like a small handsaw blade or even a utility knife and scrape between the concrete lines or cracks where some of these weeds grow. It really does work, I've tried it and had good results. Its not 100%, but if you keep at it consistently, it'll go a long way in weed management.

I second everyone that its a good idea to pull oxalis from the roots. Weed killer has not worked for me for this weed.

Thanks everyone. I know that weeds are a part of gardening, and we are staying on top of them as we see them come up. We are being careful to remove the oxalis from the roots even it if means removing a small part of the host plant along with it. No worries.... it was shocking at first to see so many new weeds coming up EVERYWHERE, but we know it comes with the territory. :-)