Should this have been allowed?

[BHS_STopping]

This Q&A response should help answer your question. Particularly, read the GDC's answer to question number 3:

3) There is no penalty unless the action interferes with MINIBOT DEPLOYMENT per Rule <G24>.

It is permissible to throw a game piece so that it hits another alliance's robot, unless that robot is deploying a minibot. All other situations appear to be okay though. Personally I think it's a little ungracious, but one should probably account for such things when designing their mechanism to handle tubes, especially when there are many more high-impact events that could cause you to drop a tube inadvertently.