This is competition.
It may make things tougher for the original company, but free competition in a marketplace is always good for the consumer.
If you made this illegal, you're just going to create monopolies where there weren't monopolies beforehand.
For an example - at one point in the past, I thought about making a Facebook game, but then I thought "If it succeeds, Zynga will just clone it immediately and pour millions of dollars into marketing theirs, making mine seem like the knockoff". So, I said screw that and I started something else that wasn't a Facebook game. In this case, this sort of effect helps Zynga develop a monopoly.
In making something, making the right set of design decisions is vastly harder than implementing it once those decisions are made. Back when implementation was also extremely difficult, this was probably less of a problem, but now that you can outsource your infrastructure, this is potentially a very real problem.