Are there any reasons why domain speculation can have legal implications?

Depends really what you mean by domain speculation....

But if you are trying to purchase domains because their name is similar to another well known name/domain, for the purposes of "stealing" some of their traffic then you might find that someone will take you through the dispute resolution to have the domain handed over to them.

A lot of people see this as similar to Cybersquatting or Typosquatting...