It's a free market. GM's are free to do whatever they wish. Bottom line is that UFA's just cost money, not players. Some signings suck such as Seabrook's, Bobrovsky's(we think?), Backes, and Ladd, others are fine such as Panarin's, Tavares' and Radulov's, others are screaming bargains like Eric Staal's.
There is a lot of competition to sign a desirable FA. It just takes one GM to want to "win" so badly that he overpays. With a flat cap coming up for the foreseeable future, I don't see it becoming a big problem.