Why do you think this? What metrics do you have to support this thought?
The one thing I wish managers everywhere would burn into their memory is this, "If you can't measure productivity outside of time spent in chair (looking at code), then you're not actually measuring productivity."
Productivity should be measured based on whether or not a team is meeting both their team goals as well as the team's individuals meeting their own goals. These goals need to be clear and realistic. A manager that is setting unrealistic, vague, or otherwise unclear goals is a problem.
When teams and individuals repeatedly meet their goals then you can increase the amount and/or difficulty. Or if it is an agile shop, the team should be increasing their "velocity" or whatever metric is used to determine work done, you shouldn't be left with too many empty days in a sprint. If you increase the amount of work or difficulty of work and these individuals are still looking at investment/finance websites after accomplishing their goals, then it is likely that you as the manager were under-utilizing the skills of the individuals on your team.
The other important thing to remember is that engineers get blocked. Sometimes more frequently than is desirable. When this happens, managers need to get involved in order to work with others in the organization to alleviate the block and potentially alter processes for the long-term.
The time to get worried is when you see that people are spending big chunks of time on LinkedIn or Indeed, or they start unexpectedly coming to work in attire that is one or two levels up from the normal day-to-day. The end is nigh when that starts to happen, and clamping down with poorly conceived attempts to wring more blood from the stone is a great way to accelerate the effect.
I'd never really considered it as something that meant that I would be leaving.