If a coach is upfront about what their intensions are I do not think most have a problem with them or their players trying out as long as that is why they are there. (It seems to be happening a lot more). But, for a coach to just show up at other teams tryouts to look at players and hand out cards that is bush league. Most parents have enough common sence to know these people are there because they cannot have tryouts on their own or they do not know what they are doing.
The people who do this without contacting the host tryout coaches show up for a couple of reasons.
1. They do not have the knowhow or support / equipment to run a tryout.
2. They do not have the money to rent field time or a place willing to let them use it.
3. They are not sure if their current team will stay together so they take their DD to others. (A lot of the time these coaches do this "without informing" the rest of their team)...
4. Its an easy way out with not a lot of effort needed to generate players (just go where they will be)
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An example of a coach doing it rite: I had a coach a few years back that contacted me because they lost five players at the end of the year and wanted to know if he could bring his six that was left to the tryout. He wanted to find a home for all six.
There was only three that I made an offer too. Two accepted and the other lady that was the coaches DD went with the other 3 to another organization because he felt it was his duty to find a home for the other three. (I ended up picking his DD up at 23U).
On a side note, I would not want any player/parent that would give the time of day to another team while at our tryouts...