This is a legitimate tactic that does the exact job it’s intended to. Pro teams don’t actually “block” per se. Teams will go to the front of the peloton to control the pace by riding what’s called a “false tempo“. Just fast enough so that it doesn’t look obvious that a disruption to the pace is occurring, but not so fast that they’ll catch the breakaway.
I think this tactic is all fine and dandy for races that are comprised fully of teams, however I don’t agree with teams using this tactic in club racing. Club races are mostly made up of individual riders along with a couple stacked teams. It doesn’t take much for those teams to get one or two guys in the breakaway and then completely shut the race down. This makes for a horribly frustrating and negative race for the rest of the riders. The only thing that the individuals can do is organize their own chase (which is difficult to do) but it often futile because the larger teams will just roll through to the front and disrupt the pace.
In my opinion the tactful way for teams to handle breakaways is to go back to the middle of the peloton and sit in (if they have a rider in the break). If a chase gets organized then they should let it happen without disruption. This makes the race better for everyone and it doesn’t become a numbers game for the teams.
BTW…The best thing to do if you’re in a race where riders are blocking is to try to bridge to the breakaway in a small group. This way you won’t be wasting all your energy towing the whole pack around only to have everyone roll you at the finish. Here’s a post I wrote last year on attacking and bridging (see last paragraph).